The gauge or wiring maybe the problem even if sender test good.
Fiero Gauges have a "ballast" resistor and contacts that get dirty loose etc. Dash itself can have crap contacts too.
Wire(s) to tank can have problems.
Worse, All can be constant or intermittent problems.
Note that Sender/Pump ground ends in the cabin and iffy ground don't only causes sender problems but can kill the pump too. Buying a new pump can kill that quick too.
See my Cave,
Electric Motors------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave