It sounds like an earthing problem, I would check the main battery earth as par of the course but you will more than likely find these all share a common earth, excluding the fuel gauge where the sender is the earth. (I can tell you for sure after consulting the bible which I don't have access to ATM)

When the fuel gauge goes does temp gauge, rev counter and dash warning lights go as well? When graphic display cause problems this is down to the earths usually.

Martin