Thanks, TXBill -
Did some further research after you mentioned the cigarette butt trick.
I had no idea the plug itself can be pulled out of the cap.
Anyway, being a non-smoker I used a folded up paper towel and, sure enough, the tach doesn't flip on and off at random anymore.
No more unscrewing and reseating the plug, which worked but only temporarily.
Next time, if this fails, I'll buy some tuner cleaner from Radio Shack and spray into the plug and receptacle, as I've always suspeced inadequate contact instead of defective plug.
Unless it corrodes inside, I just don't see much that could go wrong with the plug itself.
VA300SD, 81, 194,000.