IAC Valve issue

I have a Ram 1500 4.7 that the idle hunts and dies at idle. It began with a PO506 error indicating low rpms. I have replaced the IAC twice. Also replaced the TP Sensor, ERG valve and MAP sensor. Also cleaned the throttle body tested the voltages of all these components and tested harnesses for shorts. After 100 miles or so the idle goes bad and just dies often when approaching a stop or just sitting. If I unplug the IAC I hear a sound, possibly a vacuum clearing after it "locks". ?? At any rate the truck runs great for another 100 miles (miles not exact) and I have to do the same. It has been in and out of shop for a month with this issue and they are lost and are at the point of changing the PCM now. Not convinced that's it but...... Am at a total loss as well. The only feature we changed on the truck was installing a Flowmaster series 40 exaust, 1 in 2 out, to the vehicle but that only modified after the converter so no O2 sensors were changed either.Some fresh ideas would be great and thanks!