Follow the bouncing needle

[joelbert2k]

My 80 carb'd spider has started behaving strangely lately. While driving down the road at a nice steady pace, it doesn't matter how fast or slow, the tach needle will suddenly bounce up anywhere from 500 to 1000 rpm and then drop back down then bounce back up. This is happens continuously and makes the tach essentially worthless.
The engine does not miss or stumble at all so I believe this is in the tach itself. I have cleaned the terminals on the tach and coil as well as the connectors behind the dash and the ground pod on the fender.
As a test I hooked up a "tune up" tach and it is nice and stable. I also made some jumper wires and hooked the tach directly to the coil, it still jumps.
I am reluctant to gut the tach looking for problems 'cause getting that ring off looks very difficult, to say the least, but if nobody has any ideas that's probably what I'll have to do.
So, the big question is "Has anybody ever seen this before?"
Thanks.

[So Cal Mark]

sounds like you need a new tach. No need to take the ring off, just remove the cluster to get the tach out

[joelbert2k]

Oh, I've got the tach out of the dash already. I just can't accept that it's not fixable.

[So Cal Mark]

used tachs are cheap though, so the investment in time and parts may not be worthwhile

[mbouse]

used tach's for 1980 Spiders are very reasonable.... name your price, pay the shipping and i will send one tomorrow.

[joelbert2k]

Thanks for the offer Mike. I already getting one from Fiasco Steve.

[Flyingthere]

We had a couple of cars in the Houston area that had issues with jumping tachs and it turned out to be a problem with the voltage regulation from the alt. the Power was surging casuing the jump. If the new tach does not work I would get it checked out.

[joelbert2k]

Thanks Bob. I hadn't even considered that.