LOW Idle when warm after h igh speed run.

88 GMC K1500 w/ 5.7 EFI
Fire the truck up, cold idles great, runs good until I maintain 40+ down the road then the idle is around 500 or less RPM. Turn the air on and walla
stall. Shut the truck down, and fire it back up and idles. Weird part is it passed smog while doing this as well..... With VERY low scores. It just doesn't want to seem to idle up at "idle" or w/ AC on at Idle. I changed the IAC (Idle Air Control) on the EFI, the old one was sludged up with what looked like old motor oil. Figured that had to be it..... Nope.
Any other ideas?
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