Jimmy 4.3l idles rough cold, ok hot

My 98 GMC Jimmy w/4.3l idles great when cold, bad when hot. At cold, the RPM is steady 700-720. When hot, idle drops to 500-600, and can
drop as low as 400 with a stumble (idles is not even speed). To add to the mystery, I can max in the idle screw but the engine does not speed up (I can screw it all the way it - and watch the throttle arm move up - but no difference in speed).
I have checked the O2 sensors, they look fine (monitored them on scope). I have checked plugs, wires, cap, rotor, all ok.
I replaced the IAC valve (at 180k why not), and also the TPS (i was suspicious). I've now replaced a lot of parts...and the problem remains. As well, when idling hot, switching A/C on and off makes no difference to idle, so I think that suggests the IAC is working fine.
Where to next? I though EGR valve for a vac leak, but wouldn't a DTC be set if the PCM didn't read the EGR's position as commanded?
Any ideas? This has been going on for over a year...
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