Vertex Nano K8600 sudden shift in X direction

I build the vertex nano K8600 printer and use the Vertex Nano Repetier V1.6.2 program.
My first object is the “Print-in-place and articulated F14 Jet Fighter with Improved Wingdesign” by agepbiz September 30, 2021
About half way and at about 3/4 of the print proces there is a sudden shift of about 2 mm in the X location. This phenomenon repeats at other levels when starting a new print.
I watched the process carefully and i could not detect any obstruction.
What happens is a sudden vibration noise and immediately the shift in X direction, so the rest of the print is shifted relative to the print sofar.
What can i do to investigate the cause of this problem?

No reaction here sofar, therefore i read all the stuff and I tried all the given improvement tips like:
1 re-oil the 4 bars
2 unscrewing the 4 bar guides a little to ease their position
3 making a higher temperature in the room
4 putting the nozzle closer or further away from the baseplate
5 cleaning the baseplate finally even with aceton
6 marking the pulleys position relative to the axis positions

All of this did not make clear what the origing of the shifting is and it not improve the occurrance of the misterious sudden shifts with the brumming noise of the motor.
So i decided to bring the printer back to the shop to collect my money back.

I 'm going to buy another mark printer.

I wish all nano users a lot of luck and succes