New car, new problems; 76 Duster 318

OK, as I said, ground the temp sender wire and see what it does. MAKE SURE you are satisfied that the fuel gauge works

Assuming the fuel gauge is OK, your top suspects with the temp gauge are:

Bad sender, easily found by the "short the wire" test.

Broken wire inside the temp sender terminal

Bad connection in the bulkhead connector

Loose/ corroded nuts on the temp gauge

Of course could be the gauge.