Temp Gauge Working Intermittently - Suggestions?

Think in terms of the "end to end" functional circuit path

Sender........can be bad, any of them can, right out of the box
Sender wire connector........can be broken inside the molded wire junction
In through the bulkhead connector-----that terminal can be corroded/ dirty/ loose
To the PC board connector. The connector itself, or the board pins CAN BE LOOSE where they are riveted to the board. Clean them up and solder them.

The gauge nuts are "fake" sheet metal deals, replace with real nuts. The PC board can be corroded where the studs/ nuts make contact. Loosen / tightn the nut a few times to "scrub" it clean

The gauge itself can be bad.