Shmoo,
Usually a problem gauge will show full all the time or empty all the time. To read properly up to half is strange. To prove the gauge, go to the sending unit wire at the tank and pull it off. With the key ON your gauge should swing to FULL. Take that same wire and ground it to a good clean ground on the body or tank. Again, with the key ON the gauge should now swing to EMPTY.
If all this happened, your gauge is good and so are the wires associated with the gauge. This means the sending unit is at fault or that the tank might not be grounded well. It will be interesting to hear your results as your system seems to be working properly...but as you stated, pessimistically! Keep us posted.
Stuart
