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If it isn't the PSU, it's a well known and documented problem with the G25 and G27.
If you have a look at my page you'll see I had the same problem and it's easily fixed.
There is a small black wheel attached to one of lthe wheels motors, it's an optical encoder to tell the game what position the wheel is in.
What happens is that over time the little wheel starts to slip on the motor's axle and hence the whel won't calibrate, you need to take the top off the wheel and apply some glue to attach the encoder wheel to the motor axle, really simple.
If you google it you'll even find close up pictures of what your looking for.
Hope this helps