2000 Mercury Sable rough/jumping idle

Ok, so here's the story. About 2.5 months ago I went home and hurt my right knee, so I could not drive and did not start my car for approximately 1 week. After that week was up, I tried to get the car started, and it would get revved up, but then die out, so the idle would not work. I had it towed and the dealer replaced the "IAC", which I believe stands for Idle Air Control valve.

This is how it is acting now.

1. Startup fine, engine revs and will idle somewhere between 800-1100 RPM in park.
2. I will drive around and get to a stop light. While stopped, the car will slowly decrease down to approximately 600 RPM. It will then proceed to decrease down to as low as 500 RPM and jump back up between 500-600. I can really feel strong vibrations coming from the engine/belt, and it seems (even though this shouldn't be the problem) that the engine wants to die out whenever I am idle.

Any ideas?

The first thing I wouldl look for is a broken or leaking vacuum hose