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...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.