Help!

c-gowik

Hello all,
I hope someone can help me with a problem. The robot lose the positions during the welding process, therefore are the seams nok. There are no messages on FlexPendant and all axes are calibrated. The result of TCP check is also nok (directions y + 5 mm, x - 3 mm). The mechanical and software positions of calibration are ok. The robot is a IRB 1600. For a interim solution I teach the tool with the 4 point method again. After the tool teaching seem all to be alright. It happend twice in the last month.
Where is the fault?