Idle Drops

[MLD]

I have a 1999 K3500 with a 7.4 and the idle is steady at only about 550 rpm in gear, but when I put it in park (after it's warmed up) and let it idle it will drop to almost 300 rpm, then up to 600 and back to 550 every few seconds. When it drops it almost sounds like it's going to die. The shop I use cleaned the air induction system first , then replaced the IAC motor, but I still have the problem. They are talking about replacing the PCM next. Any thoughts on whats wrong?

[GM-Tech]

Sure sounds like a dirty throttle body. I'd check it myself even though they said they cleaned it. Never seen a PCM cause that kind of a problem though anything is possible.

[DejectedHalo]

Even though a different vehicle, I had an RPM flux. Cleand the TB and all is well now. A toothbrush works well as for reach and removing the buildup.