Help! Strange 3.0 V6 problem - 90 4WD pickup

I have a 90 Toyota 4wd xtra cab PU with the 3.0 V6. The truck (once warmed up)
idles at a normal 800 rpm. After driving and letting it sit for 10-15 minutes,
it only idles from 200 to 500 rpm depending on the ambient temperature. (in the
summer it the worst - 200 rpm.) After driving for 5-10 minutes all is fine.
What gives?? There are no check engine lights, truck only has 65,000 original
miles. The head gasket recall was performed at 60,000 and it did this prior to
the head gasket job. It seems like a temp senor or computer problem to me, but
it has been to two different Toyota dealers and no ideas or solutions. The
truck did not do this when it was brand new. Thanks to anyone with some insight
into this one!