Oh, I know this one (well, a little anyway). We had that same problem with our old stove, about 7 years ago now. We thought that everything was just taking a really long time to cook, and we went out and got a thermometer and it said 250 when we were supposed to be cooking at 350, and we thought the THERMOMETER was broken and took that back to the store! (Duh!)
It wasn't a thermostat problem, and I can't remember what the problem was, but it was very very fixable.
We had a repair contract on our stove via PSE&G (and we used to have one through GE) and the guy came out and fixed it in about an hour. Heating element maybe? That stove eventually died, but this was not the cause of death. Good luck!