Quoting fuel:
did you check the basics like throttle plate close adjustment - does the throttle plate close completely partially jamming up or is there complete smooth movement from closed to open? Check there isn't a build up of muck around the outside of the throttle plate further sealing off the flow.
My guess is the throttle plate is hard closed, the base idle screw is not allowing much bypass air through, and the ISC is at max adjustment when warm but the FIAV is providing enough bypass when cold to keep it from stalling.
What I would do:
1) remove throttle body to make work easier
2) clean throttle body and throttle plate
3) adjust throttle stop so throttle plate just closes but doesn't jam close, and then set TPS
4) refit throttle body and start engine and get up to temp (fully warmed up) then
5) perform base idle screw adjustment by grounding the base timing connector on the firewall and the diagnostic connector pin by the fuse box (check
www.vfaq.com for this) and adjust base idle screw to desired rpms (750-800)
6) hopefully enjoy a stable idle hot and cold
I think the solution is going to be here. I had a similar issue when I swapped my throttle body. Good warm up idle and good once warm unless I was using accessories with significant amperage requirements, which dropped the idle to about 600rpm. When driving, the engine would stall if I let off the throttle very suddenly when running high rpm and/or if I was stopping while actively braking and using the lights, heater, stereo, etc. I would literally have to plan before every stop, slowly decelerate turn off heat, stereo, check temp and turn off cooling fan if possible, brake prematurely while relying on downshifting to do most of the work in an effort to keep the engine running. It was terrible, I hated driving.
The biss screw was almost all the way closed on the swapped TB. While cold the isc had plenty of room for movement to deliver good idle stability but when warm it was fully open. Trying to compensate I adjusted fuel settings which made the car run through the stops but adversely affected the cold idle. Which led me to the beginning of idle tuning and eventually the biss.
The odds overwhelmingly point to a problem with air delivery/metering over electronic failure. Many of us, including me, skip the tedious steps of checking for boost leaks and proper operation of the throttle body and it's related adjacent components like tps because they SUCK! Every time I or one of my local friends with 4g63's had idle issues it turned out to be either a leak somewhere after the air metering device or improper or malfunction of the TV or something connected to it. Being critical of your own work ethic when mulling through the basic idle tuning crap pays off.
OH, I have a 90 GGSX turbo non-Eprom ECU I'd be willing to send you on loan if all of the basics are checking ok.