Make sure its not electrical first, check the simple things like fuse connection [swap it for another fuse even], also the wiring to the gauge from the tank, and the plug behind the front dash etc
If its a float, it usually stays bad, doesn't change or jump around. I'm pretty sure its a tank out job, I've never done it, but I've changed fuel pumps a few times and can't see how you could get at it from the top. Somebody else may confirm????