My 1970 ss 396 tach operates intermittently at best and when it is operating it will bounce around when it gets above 1200 rpm.
Any ideas on a fix or at least a trouble shoot.

Just a question. Do you have HEI or Points still?

If your still working with points check the connection at the coil / dist. clean it and put it back on. You could have an open condenser or a condenser going bad. Its a little capacitor used to help with RF. If your working with the HEI I'd still check connections at the dist. I would be inclined to think there was a brake in the wire somewhere between the gage and the connection at the ignition or a bad / dirty connection.

I would suspect the gage being bad last but certainty not rule it out.

Just some thoughts.