Hey, I have an '83 Trek 400 too! It's my rain bike and heavy as lead but I still like it. Mine has been upgraded a couple of times from my parts bins and still works well.

Your Sun Tour rear derailleur is 22 years old and wasn't top-of-the-line to begin with. Do you expect it to last forever? My guess it that the pivot wear is beyond simple clean and lube. Spend $20 and replace it with a new one.