Fanuc Robodrill Z axis variation

Hello, I have a Fanuc Robodrill machine center, this machine do a milling face in Z14.0. This program works well but the machine give me a variation of 50~400 microns in 1 or 2 pieces, then it get back to the right position and machine good around 100 pieces then vary again.
I have checked:
Endless screw axis Z. OK
Linear guides of axis Z. OK
Servomotor amplificator. OK
Table lineability. OK
Clamping unity. OK
Spindle conditions. OK
Axial movement. OK
Encoder. OK
Ins and outs card. OK
Piece support. OK

If you have any idea about what the problem could be, it will help me so much. Thank you for your time.