I had the same problem for like a year, but I finally realized a few months back what was causing it. My TH-fix plug was drilled out too large. After replacing the IAC, cleaning then replacing the MAF, and having no luck, I decided to go back to square one. I took out the TH-fix and started over. I drilled out like 6 different size holes and tested each, then I finally figured out what worked perfect for me (I forget what size now...but it would vary by car anyhow). Since I did that my RPMs have not risen by themselves AT ALL. It's such a huge relief to have fixed that problem. It was so bad and annoying that it got to the point I just wanted to get rid of the car! If you have the TH-fix installed, take it out and maybe make your hole a bit smaller. If you don't have the TH-fix installed, give it a try.
TH-Fix 
