I do hope this is the right place to post problems... :/

My problem is that when after a while of driving, usualy around a half an hour or so my car will suddenly shift itself into 2nd gear, regardless of what I'm doing, thanks to my daily commute I'm usualy going about 50 miles an hour when the car suddenly decides that 2nd gear is a good idea the rpm's will skyrocket to 3000 per minute and the car lurches. It usualy switches back and forth once or twice after the first time, then it will stay in 2nd the entire rest of the drive, regardless of how fast, or slow I'm driving even if I'm at a stop light, I will have to start from 2nd. So far the only thing I can do about the problem is stop the car and restart it.

Info on the car, 1998 Mitsubishi ecplise Spyder, its got a 2.4 liter engine
I do know that the EGR valve needs a cleaning, but I was waiting until it got warmer to do that... which also reminds me that I have no idea where thats located... other than that I've got no idea why the car is doing this, so any help would be much appreciated, please and thanks~

edit: oops... oh yeah its an automatic...