When I start my car, its just fine, when I begin to drive it, it rev's really high and the maximum speed I can reach is 30 mph. After about 4000 ft it stops and the o/d light starts flashing. Now I know about the over drive button on the side of my automatic gear shift, however when it is set to just regular drive it will sometimes it goes back and forth between overdrive and regular drive and I don't know why. This is not something that goes on and stays on, but it will happen about once in any given drive for sure, but sometimes while I'm driving it will happen again and it will go off, other times it will go on for a second go off and go back on etc. I am wondering if it is the transmission or the throttle position sensor or some other third thing.