Hmm, so only after warm up... Good news is in my opinion it's nothing major. Bad news is it's going to be a PITA to find. What I think is happening is that you have a air leak around or through the throttle body, allowing air that is unmetered into the combustion chamber. The computer thinks it's getting x amount of air and tells the injectors to squirt y amount of fuel into the cylinder to maintain a preset air/fuel ratio. The problem is you're actually getting x+? amount of air now and the computer is only injecting y amount of fuel still, which throws of the a/f ratio, causing a lean condition, which causes high idle. Check any rubber hoses, vacuum lines around the throttle body to look for sings of dry rot, cracking, looseness, etc. if nothing found, get a bottle of starting fluid and LIGHTLY spray in very short bursts all around the throttle body and hoses leading to it. If you hear an increase in rpm, you've found the general area of the leak. Do not spray near the air filter. If this yields nothing, bad throttle body seals, that don't seal as tightly anymore seal even less when the metal around them gets heated up and the bore they reside in expands. The seals are replaceable, but are evil little buggers to do, I'd go junkyarding for a used throttle body and replace the whole thing if one can be found, it SHOULD be easy to do. Only other option, AND the most expensive probability is the mass air flow sensor, easy to replace, but not cheap, junkyard used if possible. If that doesn't do it, ecu, but as said before, highly unlikely. Hope this helps.