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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.