No, really, you have two quick places to look - at the throttle plate / body, and at the idle / WOT switch.
  If the throttle plate is worn alot, it may not close properly any more. It gets trapped in a partially open position, sometimes. This would be #3 from your list below.

  If the TP switch is on the ragged edge of adjustment, the difference in temperature may be enough. Or a worn throttle shaft bushing that allows the switch cam to move out of position. Or the switch itself may be dying in the "hot" conditions, after running a while.

  There are lots of other possibilies, too, so don't stop here...

Jeff Clayman <jclayman@MASSMED.ORG> wrote:
  Hi,

I have an idle problem. Van starts well and makes it through cold idle but after running for 1-2 hours the idle lingers up around 2000 rpm and doesn't drop down to 750 or so in drive at a stop making the van hard to stop. Strangely, sometimes if I let the van idle for a period at the high idle it does drop down and starts wandering around. I replaced the engine about a year ago and initially there were no problems. I didn't replace the oxygen sensor (original van had 87K on it) and I have a lingering suspicion this is the problem. Also, the idle seems to wander around 400-500 rpm or so when in neutral when the van is warm. No starting problems through. Van does not stall.

Any votes about where to start.

I bought Probst's Bocsch FI book and he recommends following:

Engine idles too fast

1. accelerator pedal, cable or throttle binding
2. Auxiliary air regulator faulty
3. Air leaking past throttle valve.

Jeff