Is my ECM Toast?

[gbmidyear66]

I seem to be in over my head here. I've been chasing a SES light that comes on intermittent (no codes set) AND an erratic high idle problem for over a week. After checking many possibilities, I am coming to the unhappy conclusion that I may have an ECM problem. Here is what I have done/eliminated:

- vacuum pump does not cycle
- no apparent vacuum leaks, all hoses on tight
- engine stalls (as it should) if I block the TB air intake
- TPS sensor reads correct voltage at idle, and voltage values cycle up/down smoothly with throttle position
- MAP vacuum hose is clear and connected, and voltage values from sensor (from scanner) look good
- replaced my custom PROM with the factory AUAH PROM (just to eliminate the custom PROM) - and strangely - the CAGS function no longer kicks in like it used to (the CAGS light flickers once in a while), and the idle and SES behavior is the same as with the custom PROM (it doesn't hang high then drop down after you come to a stop - like it did before with the AUAH PROM)
- when I can kick the idle down, it will hunt around 900 to 1100, won't come down to 750 like it used to
- disconnected and cleaned the ECM connectors at the harness with spray contact cleaner - no effect
- throttle body is clean, throttle cables move freely

- IAC is clean, and I know it is not "dead" - because if I disconnect it, the idle gets much worse (stuck hard at about 2,500), can an IAC go "bad" but not "dead"?

- Should I swap the IAC? ( just in case), or does everything point to the ECM?

Glenn