Same problem, I'm afraid. It drove me back to using a PS3 controller. Thank goodness MotionJoy shelled out to get their driver signed in the meantime.
Having spoken to the company, they said it was a hardware fault - which I'm inclined to believe, especially given the way the problem 'spread' to the other thumbstick over time, too, not to mention generally just got worse. Calibration adjustment was a temporary fix at best requiring ever higher levels of compensation.
A shame, really; it's a rather nice controller. I'm still considering replacing it.