Does the engine hesitate at all when very cold? Does the temperature gauge read different from the way it did before, assuming the engine had a fast idle some time in the past (different from the previous reading when either cold or hot)? If either is true, the ECT (engine coolant temp) sensor may be out of spec and not providing enough cold fuel enrichment.

If you turn on the air conditioner or lots of accessories, does the engine idle slow down a bit but then return near or to the original reading within a second or 2? Similarly the idle should jump up a bit then return, as you turn off these extra loads. If this does not happen I would suspect a dirty or damaged IAC (idle air control) valve.

Otherwise, I would do as Klaus suggests and also check that the throttle body bore is scrupulously clean and the linkage and cable are not at all sticking.