just to rain on everyones parade that is having this problem, I don't have anything like this going on in my car. Almost every morning I leave my apartment with about 10 minutes to drive to campus, find a parking spot, and hop on the bus to class. I don't have the time to let the car warm up, and lots of times coming out of the apartment complex it is pretty busy, so I often have to accelerate hard after the engine running only about 20 seconds. I have never had it act any differently than it does after it's been running for a long time.

I wish I had some suggestions on what to check, but everything I can think of has already been covered. (I know that changing the air filter has been mentioned, and I found with my K & N filter I have to clean it every 5,000 miles, yes five thousand not fifty thousand like they say, or else the performance starts to drop.)

I also have some weird stuff going on with my mpg. No matter how hard or gentle I drive, whether it be a 700 mile interstate trip, or racing around town, going to autocrosses, my car NEVER is more than .5 mpg off from 21. I don't mind this much as when I drive my mom's sentra it usually gets around 24 if I drive it the same way I do this one. (my mom gets around 36)