Strange Y-axis 'skipping'

Not sure if this is the right forum for this question as I am using the K8200 in CNC milling mode! However, since I suspect the problem would be exactly the same in extrusion mode I will continue…

There have been a few posts about ‘skipping’ in one axis or the other, and several people have suggested sticky spots on bearing surfaces or microswitch problems.

I don’t know whether my problem is similar or subtly different, but it is completely repeatable, so unlikely to be either of the foregoing.

When doing test runs, the X and Y home buttons in Repetier will work correctly. Then, without warning, pressing the Y-home button simply moves the bed about 5mm in the +Y direction. The position indicated on-screen is 0.00. Pressing Y-home again gives another +5mm and so on. The usual + and - movement of the home function does not occur - just a move. X-home works perfectly normally. I cannot move the bed in the -Y direction (because the software thinks it is homed!)

I have updated to the latest (very different) version of Repetier Host but the symptoms are exactly the same.

I can usually cure the problem by removing the USB lead, waiting 10 seconds or so and then replugging the lead. Things then work normally. It can’t be just an intermittent lead as this would not account for the repeatability of the problem. It’s definitely nothing to do with sticky bearings or microswitches.

Anyone any ideas?