I had similar problems and what I did was this, I see that you have already made most of it but can’t hurt to list it, as I already wrote this up in Swedish for another person with the same problem. This is just the translation:
Sorry if a lot of it is of no use to you, but here we go 
In the end for me it turned out to be a faulty driver board for the stepper causing the problems. Sometimes it would simply miss steps all together.
This was manifested by very large voltage swings when measuring the calibration voltage of the driver. Stupid of me not to measure in the first place… But it could just as easily be that the calibration voltage is correct but the driver fails in other ways, be sure to try and swap them (see Wiki for calibration: k8xxx-3dprinters.crimed.be/w … er_drivers).
- Remove all belts from the XY-carriage clamps and make sure everything runs smoothly. When belts aren’t attached the printer head should pick up speed so fast that you feel you need to catch it before it hits the end if you lift one side of the printer only a few centimeters. If you need to lift it more than five something is definitly wrong.
The manual said to not apply too much oil to the rods, in my opinion it’s only too much if it drips off. Be very liberal with oil but try to avoid getting it on the belts. I.e. as much oil as you can without causing any drips. Get some proper paper that won’t give of any residue (not household or toilet paper, I said proper…), soak it in oil and wipe down all rods. Do this regularly. Also, don’t use WD-40, semi-dry PTFE lubricant or other similar products. Use correct machine oil made for the purpose (also known as sewing-machine oil).
If it doesn’t run smoothly follow instructions in the Wiki for adjusting (k8xxx-3dprinters.crimed.be/w … _Alignment). And don’t forget the oil
2. Tighten stepper motor belts by moving the steppers as far down as you can. I made these belts very tight and have seen no problems in doing so. Make sure you have someone to help you with this. Unless you have three hands so you can hold the stepper while tightening the bolts I don’t think it’s possible to get it tight enough. You have to get it both tight AND properly aligned (axis perfectly parallel to the bottom). The alignment is very important in eliminating belt rubbing. I used a construction cross-laser and some additional help from a stone marble on the bottom to first align the printer and then put the laser inside the printer to align the steppers.
3. When bolting the belts back to the XY-carriage my advice is to not bother trying to adjust the carriage rods by measuring distance to the wall. I attempted this a few times but it only caused issues forcing the carriage into a position that wasn’t well aligned with the supporting rods. In the end I first temporarily tightened all the pulleys in what I deemed was a good location, attached the belt holders, then vibrated the table the printer stood on (using an impact drill on one of the legs) to let the carriage fall into its “natural” position. I then torqued the holders down, taking great care holding the carriage still while doing so. In the end I’m not sure how much all this vibrating and messing about helped, but when measuring after tightening the carriage is less than 0.02 out of alignment at the worst place. Might have been just dump luck though… If your alignment is good as it is I wouldn’t recomend trying to redo it
4. Make sure all the belts are very well aligned by adjusting the pulleys. If your temporary locations where decent it should be good already, but make sure to get it 100% now. For me the rubbing of motor belts to the longer belts caused problems with incorrect steps. With careful adjustment I was able to almost completely eliminate the rubbing which improved print quality greatly. When all belt pulleys are very well aligned, make sure to bolt them down using bolts in both holes so they stay aligned. When you have made a few prints successfully be sure to remove the locking screws (one at a time!!!) and apply Loctite to secure it in place. Make sure to use a Loctite variant that can be removed with limited heat input.
5. After all these steps my general print quality was way better than I expected for such a cheap machine, with no modifications from stock, but sometimes I would still get a weird shift in one axis. At this point I was confident that the problem didn’t derive from bad setup of the machine so I turned my attention at the steppers, and sure enough, one of the driver boards was faulty.
Start out by measuring the calibration voltage of each driver as detailed in the wiki: k8xxx-3dprinters.crimed.be/w … er_drivers. If all of these are within specification swap drivers for the X and Y axis and run a few prints to establish correct operation of the driver. If your Y-axis starts to behave badly you know there’s a driver problem.
6. If you still have problems with pure X/Y shift despite properly doing everything above you should try swapping X and Y steppers to make sure the problem isn’t in the stepper itself.
Z-axis
If this takes care of your XY problems you should also spend some attention on Z-axis. Make sure the (crappy) stock bearings runs as smoothly as possible by disconnecting the coupler and repeatedly running the Z stage assembly up and down (100s of times, be careful not to hit the nozzle). Apply generous amounts of oil to the rods. If smooth operation can’t be achieved this way (it should definitely fall down on it’s own at an alarming speed, but might still be a tiny bit jerky in the start due to the bad bearings) loosen the bolts for the Z-rod caps (Z ROD MOUNT 2) and the Z-rod fastening assembly (Z ROD MOUNT 1). Move the stage up and down repeatedly until it runs smoothly and then re-tighten the fastening assembly (Z ROD MOUNT 1). Move the stage some more to ensure it still runs smoothly and then re-tighten the Z-rod caps. The Z-Axis should now be smooth.
Re-attach the coupler and establish that it’s easy to turn the threaded rod by hand. Apply a generous amount of machine oil to the plastic nut but be careful not to overdo it as you don’t want oil in the stepper. Still, not using enough oil is probably just as bad as it will render worse prints with more Z-wobble and prematurely wear the plastic nut down. It might also but excessive strain on the stepper, causing problems with large currents that might damage both the stepper and the electronics. Perhaps not that plausible but it’s a possibility. The rod and the nut seems to be machined to relatively poor specifications so the assembly needs all the help it can get to ensure smooth operation.
Extruder
The Velleman firmware isn’t correctly calibrated for the length of filament it feeds for a certain requested length (not even the latest version). This may work, or it may cause problems. Velleman has compensated for the badly calibrated, overfeeding, firmware by setting a lower filament flow in the Repetier-Host software but it still isn’t 100%. Re-calibrating the firmware will probably save a lot of grief down the road and for me it gave an instant improvement in edge quality of the printed parts.
Temperature
I use Velleman PLA and the high default temperature (210C) in combination with the overfeeding extruder caused way to much hot filament to be applied at a time. This caused components to “sag” in the middle of large areas and it made sharp edges and small holes impossible to print. I dialed the temp down to 190C, but this is subject to constant adjustments. For e.g. the first layer (increase to ensure bed adhesion), very compact parts (further decrease to avoid sag), parts with lots of holes and tiny detail (increase to ensure self adhesion). Note that I don’t have a heated bed.
After all this adjustment I’m now able to consistently print high detail objects at the “Fast” speed of the Velleman default profile with no problems what so ever. All details are sharp, holes are printed well, circles are circular, Z-wobble is as close to non-existent as I dare hope for etc. etc.
