Engine Surges in cold weather

I have a 1990 legacy and lately the engine has been surging whenever the weather is cold. I've checked the voltage output and it is fine. It surges whether the engine is cold or at normal operating temperature. When I do a dianostics, all codes come back ok. Any ideas as to what the problem could be?

My first thought is a vacuum leak. Second thought is the IAC (idle air controller) neeeds cleaned.

Nick