Unstable RPM

Model: Pontiac 6000 LE
Engine: 2.8 L V-6 MPFI
VIN: W

Last week when I was in the parking lot, my car's battery was completely dead, so I changed the battery and now the RPM is strange. When I start it, its fine, but when the engine warms up a bit, the RPM drops. For example, when I stop at a stop sign, the RPM goes down to 500 and I'm not using the A/C system, and when I do use the A/C system, the engine doesn't rev a bit higher, it just revs even lower. It has stalled once already. I cleaned the intake and the fuel injectors, but this didn't help at all. I also did the Idle learn procedure, but that didn't work. I think it could be the IAC, but I'm not sure. It always sounds like it is ready to stall each time the car is stationary. Please help me with this!