I second the catalytic converter.

Easy test : run the car for a bit a night (until it gets warm and starts exhibiting the symptoms) and then run it a bit more. Look under the car for a red glow. If you see it that's your problem. Fix=replace cat. A friend of mine started a brush fire with a hot converter in a field when working as a surveyor. Unfortunately the car was in the middle of the fire and didn't do well.

-Zeke