Bouncy tachometer
 

Tach goes kinda crazy at about 3k or so, then it just bounces all over the gauge, any ideas??

Any common problems like this??

The problem is when the solder in the back of the tach starts cracking. All the soldering points have to be re-soldered and it should work like new. There is a write up here:

https://www.rx7club.com/3rd-gen-archives-73/fix-tach-17247/

good luck.