the symptoms you describe will have a different meaning depending on whether the engine has or hasn't got an O2 sensor and does or doesn't run under closed loop control.
But anyway the first thing I would look for is an air leak into the inlet manifold. One method of doing this is to use a flammable spray of some kind near potential leak points such as rubber joints, gaskets, hose connections etc. when you hit the leak, the idle speed picks up.
If there is an air leak this may be tolerated when the engine is cold for two reasons;
1) the mixture is rich anyway and
2) the amount of air going into the engine is quite large; the drag of cold oil etc in the engine means there is usually at least twice as much fuel being burned at idle on a cold engine vs a hot one.
The latter point means that the significance of any air leak becomes greter when the engine is hot.
cheers
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