Chrysler Repair: Engine revving by itself, 2000 chrysler 300m, manifold vacuum

Question
2000 Chrysler 300M. 124,500 miles. Very well maintained. Many new parts. Regularly serviced every 3,000 miles. In mid-June I started the car and the tach went to 1,500 and stayed there. I shut off the engine. Re-started and idle was normal. Didn't happen again for a month. Then started happening again about 2 weeks ago. Only when I first start and/or have driven a while, parked, and re-started later.  Doesn't happen every time. July 27 had the Automatic idle control motor replaced and throttle body cleaned. The next day same thing happened. It seems to happen more often with the A/C on. No computer code shows on diagnostic. No check engine light comes on. Other than this the car runs perfectly and still gets great gas mileage. Any ideas?

Answer
Hi Bill,
My first thought is that there is a misreading by the pcm as to when the A.C clutch is being called into action, as that as I recall is programmed to raise the idle speed to account for the drag of the compressor. But why that fault occurs, if it does, is not obvious to me. I will think about this some more. It could also be an intermittent intake manifold vacuum (air) leak which is sensed as a too lean condition by the O2 sensor and correspondingly lengthens the fuel pulses via the feedback loop.
Roland
PS Sorry for the delay but I just found your question in the 'pool'.