Fuel gauge finally works, but only briefly

Principle of operation should be about the same.
Bet its not terribly accurate but poke around the b-body forum and moparts to see if thats a common problem like it is with the A-body senders.

Anyway.
It sounds like its losing connection to the sending unit or the sending unit to ground.
Another possibility is the something in the feed line is opening. There no fuse so it could be the IVR opens and doesn't shut???

Set up each gage with a reostat or a set of resistors. About 11 ohms, 23 ohms and 70 something ohms if I recall correctly.
If I recall correctly your car had a standard instrument panel. So only two gages. Temperature and Fuel.
Test each gage to see if it works when connected to the resistors. If the gage works, the problem is downstream.
Makes sure neither wire to the sending unit touches ground.

edit: Make your own gage tester
https://www.forabodiesonly.com/mopar/threads/gas-gauge-74-duster-help.407869/post-1972086136