Looks like I will have to get back to this game (but I do have an M3!). I am out of town until next week though. My username is the same as here.
Lucid, A friend of mine lent me a logitech wheel a couple of years to try with GT4, I had the same problem you describe, sounds like it is the wheel. I just bought a wheel for myself a little while ago and do not have any problems at all.