Sounds like you got some of basics, but, I didn't see anything about replacing vacuum hoses. After 20 years the vacuum hoses might need replacing. replace them all, vacuum hose is pretty cheap. Do them one at a time so you don't forget what goes where. If that doesn't help your carb might need rebuilding.
From the symptoms (especially inconsistant idle timing) sounds like a vacuum leak to me. Could be a hose, or, the carb. If these don't fix it you might need a new distributor, could be the bushings are worn and I don't know if there is a way to replace them, I'm sure there is if you are dedicated enough... (do you have a dwell meter? if so are its reading constant? - usually if the distributor is worn both timing and dwell will bounce around)