II also have this issue with all three of my Pluses (which never gave issues until now), and recall a similar issue surfacing with one of my 512K's nearly 15 years ago. Never bothered to deal with it back then, but now that ALL my pluses are giving me problems I decided to google the symptoms and lo and behold there is a thread on 68k!

Might pull the analog board out of my original Plus which I partially recapped in around 2003 and see how i go.