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.