z axes looses about 0.01mm when moving down.

I soldered all the connections directly to the arduino just to make sure there is no bad connection using the plugs.
As Christian said i tried a external powersupply to the arduino. Both without any improvements.
Tomorrow i'm gonna try a shielded USB cable and a powered USB hub.

SOLVED!!!!!!

I purchased a USB hub with external power and the problems disappeared at once..
To make sure i changed a 1.8meter USB cable with a cable of 1meter length and there were the problems again. So back to the cable with 1,8 meter.
All my problems were related to a weak USB signal and a bad USB cable.
Everything worked today 4 hours without problems and i really hope it stays that way.
Thanx everyone for your help!!