I can't tell you whats wrong but here are some things to check. The O2 sensor, if this isn't working then the mixture won't be regulated. Leaks in the air intake system are a good thing to look for, they will sometimes occure along the bellowed hose going from the air mass meter to the throttle body.

Here is an easy thing to do that should be part of your basic maintenance and can solve your problem. Clean out your flame trap and the throttle body.

The flame trap is a hose with a filter in it that keeps the engine's pressure regulated but gets clogged up pretty quickly. The flame trap empties out near the throttle body and can cause a mess in there as well. The flame trap is underneath the center of the intake manifold and has a hose that runs out to the throttle body. The throttle body regulates the flow of air into the intake manifold at any given throttle position. If it is mucked up then the valve might not close all of the way, open all of the way, make the air flow turbulent, or just get stuck.

The throttle body is located along the air intake system. If you follow the direction of air flow into the engine it goes from the air filter, to the air mass meter to the throttle body. There is also a lever arm connecting the throttle body to the throttle pully. You should clean this out with a bottle of techtron so that what ever solvent is left inside will not harm the engine or catalytic converter. Also, you will need a new gasket for reasembly.

Good luck, I hope this helps.