The fact that it's only happening when warm is an important indicator - this
seems to suggest that it's only idling improperly when in "closed loop"
mode.  There are two modes of operation on a fuel-injected vehicle:
open-loop and closed-loop.  When in open-loop mode, the computer determines
the amount of fuel to inject based on a fixed table (called a "fuel map")
based on several inputs: the MAP sensor, the TPS, the temp. sensor, etc.  In
closed-loop mode, the computer still uses those input as a "suggestion" of
how much fuel to inject, but relies on feedback from the oxygen sensors to
adjust the fuel mixture up or down, attempting to maintain a specific
air/fuel ratio.

Since you're saying the idle only gets erratic when the engine has warmed
up, I'd suspect a faulty oxygen sensor.  If I had a scan tool capable of
displaying oxygen sensor outputs, I'd use that to confirm these suspicions.
I'd be somewhat leery of simply throwing an oxygen sensor (or two, depending
on your specific vehicle), because of the cost (anywhere between $50 and
$100).  However, this was exactly the problem with a friend's '00 Durango
recently...  erratic idling when warm, stumbling during cruise (cleared up
when he floored the throttle - that also puts the engine back into open-loop
mode), etc.  Even though there were no codes set, it was the oxygen sensor
that was the culprit.  After changing that out, his problems were solved.