Older pair of Stax Lambda Pro's (late 80's?):
there seems to be a slight buzz that's continuation of instrument
sound fading off, especially noticeable on poorly recorded music.

Any guesses to cause (plain old age[of cans!], notorious "etch" of older electrostatics, poor source/interconnects, etc.)?
Any solutions beyond major $$$ overhaul?

I've never noticed any buzzing on the recent Stax systems I've tried (Signature and Omega II). However, I've read an article some time ago where the author describes a side effect that could be your problem.

Quote :

"Sure enough, on certain strongly plucked notes, the guitar sounded as though it was being processed through a reverb unit. What I think I was hearing was the electrostatic diaphragm vibrating sympathetically with the note -- or possibly being sent into oscillation. It doesn't last long, but it's inescapable once you've noticed it.

BTW when the stator was under control (reproducing a bass tone simultaneously, for instance), I couldn't get the Stax to reproduce the resonance ripple -- it only occurred with solo instruments at certain frequencies."

Complete article :
http://www.onhifi.com/product/stax_srs4040.htm

Thanx yarinkel! I think you've hit the nail on the head!
Last night I was listening to William Ackerman's "Conferring
with the Moon" & this phenomenon happened from
guitar notes& low rumbling cello notes-but not the other instruments on album (violin, lyricon, English horn)
...
& welcome to head-fi !