Idle problems (only when "lukewarm" !!) -5K-

[Vangelis Tozakoglou <evangelo@mecf.wustl.edu>]

Hi everybody,
I am having idle problems when I start the engine after it has run for a while.
Specifically:
When the engine is cold (ie. morning) I have no idle problems at all.
When its warmed up (say after a 10 minute ride) the idle is high but stable at
1000RPM.
If after driving around switch the engine off and let it sit for 15-20 minutes
the car starts fine, idles at 900 for a couple of seconds and then goes down
to 200-300 almost ready to die.
If I run it a bit it goes up to 1000 again ...

It happened suddenly, no previous notices ...

So far, I checked for obvious leaks in the vacuum, (including the oil dipstick),
cleaned the idle stabilizer and a few electric contacts I found in my way.

Does anybody have any ideas on what can the problem be ? 

Thanks in advance
Vangelis

5000CS T, 164K miles.