I'm not a mechanic but was googling some problems with my car and it
wasn't even 5 minutes ago I was reading an article that didn't apply to
me, but sound a lot like the problem you're having. Read this:
http://www.2carpros.com/topics/idle.htm . There are 2 or 3 different
post on that one page describing exactly what you are saying. Those
persons have had their IAC and TPS changed and it did NOT fix the
problem. The answer to the problem was the below quoted suggestion.
"This is a difficult problem to detect because the computer can not
sense a malfunction, the vehicles computer system thinks everything is
ok. In reality what is happening is the electrical component inside the
MAF sensor becomes contaminated by air impurities. These containments
imbed themselves to the electrical component that senses the air flow
causing the sensor to be slightly off, but not enough to trigger the
MIL (malfunction indicator lamp). Replace the MAF sensor with a new.
NOTE: (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on
manufacturer. It does the same thing only with different names)."