I have a 2001 Grand Prix GTP (supercharged), and lately I've been having some strange problems with it. I'm not great with cars... so I have no idea what's causing the problem, and before I take it to a mechanic I would appreciate some advice.
Problems started about a week ago. First problem was the relay switch for the cooling fans went, the car overheated and acted up. Speed dropped, wouldn't shift properly... etc. Pulled over to cool it off, replaced the switch, and haven't had any problems with overheating since.
I'm getting similar problems now, but the temp isn't climbing past 85. It lags pretty badly when I accelerate, it lurches when it shifts, and after about twenty minutes or so of driving the speed just slowly decreases and if I come to a stop, the car wants to stall.
Any ideas on what the problem might be?
Like I said, I'm not great with cars and I don't want to get ripped off if when I take it in to get checked out. I've also been getting all sorts of advice from people at work, and I don't know if it has any merit.

Any help would be much appreciated! Thanks so much.