[codefox1]

All old sets drift a little bit. I think you probably still have one or more carbon resistors that is high in value, and it gets worse with a little heat. At a nickel or so apiece, why haven't you replaced them all? Wirewounds are either good or bad or shorted to the chassis. It's a crapshoot with mica capacitors, usually you get crackling or loss of signal, but drifting could also occur. Of course there are more bizzare things that could be going on too.

Let's see who else has some tips.