Wow...the advice given has been pretty retarded for the most part lol. Especially the mystery oil idea. Anyhow, on top of all the things mentioned in this thread I can't believe that nobody has mentioned to check to see if your catalytic converter is "blown out" or clogged, the cat if clogged will cause the symptoms you're having. You can try running with the cat disconnected and see how it runs(Illegal in some states,possibly all) and if it runs fine you know that's your problem. You can also try banging or tapping it with your hand and listen for stuff rattling around inside. The extreme back-pressure this would put on your engine would definitely cause it to run poorly. Had this problem on my 89 Camaro Z28 back in the day.