Mine is shot, I'm going to buy a $200 arcade and then put my 120 gig HDD on it. But open yours up first and check the sensor like Jose said, it could just have some dust on it...They are easy to replace as well....
Like others have said, most likey something small. I would guess a bad connection/loose or bad ground or even the button going bad. If it hasn't been grinding or missing/waiting too long to shift then you should be fine but do have it check out!!