The guys above gave some good suggestions.
In order to know what is causing the problem though youre going to need to get a bit dirty and figure it out youself. Get some carb cleaner and with the engine running spray it around the intake manifold TB etc. If idle jumps youve found the problem.
If its a bad thermo valve when the engine is at op temp take off the FITV housing lid and stick your finger over the hole. If you feel suction then the wax pellet has become to old to expand and you will need to replace the FITV.
If none of that is the case it could be a sticking throttle plate (open a tad resulting in off-idle) leak in a vacuum line (use carb cleaner again) etc.