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.

Thanks again.

VA300SD, 81, 194,000.

I'll report back the results when the tuner cleaner is used. This has worked wonders before in older sound equipment when contacts and rotary pots (volume and other dial controls) start acting up from buildup on the contact surfaces. I've also used this stuff on power window switches.
For the time being, nice to see that tach needle register when the car is started up.
Thanks again.