Try disconnecting the wire that goes to the tank. Take a jumper and jump that terminal to ground. Turn the key on and remove the jump wire. If the needle sweeps, you have a bad connection in the wire or sender.
If it doesn't do anything, maybe try insulating the gauge from the dash? You'll probably need to run a jumper to the gauge casing for ground.
I have a problem with my 48 Olds gauge (originally 6v pos. ground) - if the casing touches the dash, it buries the needle, but I still have to use a jump wire to ground the case - I haven't figured out why yet. If you converted from 6v- to 12v+, you may be having the same problem.