I just replaced my sending unit and checked my fuel gage. It is not a hard task. If you are getting readings sometimes and not continuosly then there is either a connection failure or a bad sending unit. the gage reads when there is a signal. I would bet on the sending unit. You can test the gage by doing a continuity check across the leads on the back of the unit. This will flow current and force the needle to move. The gage is bad if it does not move with this test. The sending unit has a copper or brass contact that swings across a linear resistor and this causes metal wear. As old as these trucks are this is a very common failure point. You can also check it with an ohm meter. You should read from 90 ohms to zero accross the resistor. DO THIS CHECK WITH THE SENDING OUT AND AWAY FROM THE GAS TANK. Make sure that both of your crimp lugs on the sending unit wire is solid.
If these things all check out then you should be reading fuel accurately. Good luck.
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