G27 wheel loses alignment in game

I have a G27 wheel that I purchased about 8 months ago, now I have a problem with centering, if I calibrate the wheel it starts out fine, but after 2 or more laps the wheel starts to move to the left and after about 4 minutes it is about 90 degrees from the center to the left, tried to look at the optical encoder but everything looks fine and seems to work, after 6 times taking the wheel apart it still has the same problem, nothing is cracked or loose , I checked the server motors, the little wheel (optical encoder) etc. but nothing seems to help.

If anyone knows what the problem might be, please let me know.

Thanks,

Fred