1994 galant motor revs up and down motor runs great except the reving what can it be oxgyn sensor you have heard many things but nothing worth trying?

Run a diagnostic test, if you have a bad oxygen sensor the computer will display the code that indicates the oxygen sensor is bad. Usually irregular idle speed is caused by a vacuum leak or sometimes the AC circuit speeds the engine a little when the AC clutch is engaged. The AC compressor is used by the defroster inmost vehicles to dry the air, so just because it's winter don't think your AC isn't comming on. Simple answer, no, it's probably not the oxygen sensor. Many people have no idea what they are talking about. It could be the EGR valve, but that would typically be controlled by something else that might be causing the irregular idle speed. If it idles super slow sometimes, check for a faulty EGR valve, but most often a high idle speed is caused by vacuum leaks. TRY REPLACING THE THROTTLE POSION SENSOR ITS LOCATED RIGHT ON THE SIDE OF THE THROTTLE BODY WERE THE ACCLARATER CABLE HOOKS UP