It sounds like you know your stuff, but since you haven't mentioned the following I will.
Look at the derailleur from the rear. Is the cage perfectly vertical? That is, do both pulleys spin on perfectly horizontal axes in all gears? And do the two pulley wheels align correctly with one another? If not, something is bent. If that's the case, bend it back. It may e the dropout or derailleur claw; it may be some part of the derailleur body; or it may be the cage. If I had to guess, I'd say the cage is bent; and that's a very easy fix.
edit:
Ack! what a debacle! While I'm typing, dakl beat me to it!
Could be bent, since it was in the parts bin for a couple of decades. To be honest, I didn't even know that I had it until I went snooping. I'll check it with a straight edge today.