Odd Idle Issue (7A-FE), Idle increases in neutral - weirdest problem you will ever hear Options

[Espo]

My 96 ST Celica seems to have brought me a slight issue this winter - keep in mind this only starts happening when it gets cold.

Randomly, when I'm driving, whether it's city or highway, and I come to a stop in neutral (I have a manual transmission) my engine idles at 1200 RPMs...then climbs up to 1700 where it sits usually. On a few occasions it has climbed to damn near 2000 rpms.

Odd solution? If I punch the gas quickly it drops back to 1100 or the standard 900 rpms and stays there. Again, randomly, but it picks one of the two. Also, if I shut the car off and turn it back on, the idle returns to 900. I'm wondering if an idle speed control valve went bad, or a temp sensor, or an O2 sensor, or the fact that maybe I'm running 93 octane without resetting my air/fuel mixture from 87 has something to do with it. I do have an intake on the engine.

If anyone has any suggestions that I haven't thought of (or wants to tell me which one it is) please feel free. I can't think for the life of me what it would be - I haven't tried pulling codes but my engine light isn't on either.

[FunkySR]

could be your idle control valve.