[Bender]

Hi Chris.
I had a similar problem recently. If you have fibre or nylon washers on the bolts. The sender does not earth to the tank through the bolts. You need to connect an earth from the chassis directly to the sender unit base. Some sender units have an earth wire spade soldered to the sender unit base. Some don't. The earth wire if connected to the sender base by the bolts. You need the wire/ terminal to have direct contact with the sender base. ie no washers between the wire and the sender base.
Hope this helps and you can be out enjoying your ride.
Bender