Do you have an exhaust back pressure valve? That's not the correct name, but I can't remember what it used to be called. Anyway, it should be located right where the exhaust manifold meets the exhaust pipe at the rear of the engine, usually on the passenger side. It looks like a spring with a weight attached, and you should be able to rotate the weight by hand. Its purpose is to hasten engine warmup, but if they become stuck they cause all manner of drive-ability problems. If you can't rotate the weight, try spraying some penetrating oil on it and leave it sit overnight.
