What OT crap are you working on

That safety lever you tied down to make it run is attached to a kill switch that grounds out the power to the coil.
I'm suspecting a bad wire between that switch and the coil. That wire may ground out if it swings to just the right place. Or maybe the switch itself, or the linkage between the lever and the switch is messed up.
I'd start at the lever and inspect everything between it and where it cuts power to the coil.

 

Its a 71 Duster? Doesn't all the wiring pass through the bulkhead connector? Those are known to be corrosion collectors. Both gauges operate off ohm readings, and the wires for both gauges pass separately through the bulkhead connector (the bulkhead connector uses male and female spade terminals all bundled together into blocks of male and female terminal ends for every wire that passes through it). It wouldn't take much corrosion to cause a problem with either gauge wire as it passes through the bulkhead connector. I would start by inspecting and cleaning the wire connections at the bulkhead connection.

 

Only one of the wires passes through the firewall, the fuel gauge runs down the kickpanel and under the interior to the trunk. I'll have a look at the bulkhead connector for the temp gauge; cleaning things never hurts.