Gas gauge quit working on '88 olds Firenza!

Before it quit, the needle would intermittantly go way high above the F end. Banging on the dash with my fist above it would often bring it back to normal for a while. Doesn't any more -- stays high all the time.
Obviously there is a bad connection somewhere behind it, but what should I check for? What would make it go high instead of low? Bad ground maybe?

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