All your probelms sound very familiar to me.
My fuel guage would rarely go above 1/2. Sometimes (after filling it to the brim) it would show FULL, but the next time the ignition was turned on, it would only register 1/2 again.
The problem ..... the fuel sender unit was not earthed properly.
To fix it, you need to drop the tank
(4 x bolts, filler neck, breather, fuel supply, fuel return, and the single electrical connection to the sender)
Then remove the screws from the top of the sender .. if they dont want to undo, dont force them, use plenty of penetrating oil, because shearing these off is a real pain.
Take the sender unit out, and away from the tank, and wire brush all the rust off the upper surface and the inside of the screw holes.
Refit the sender unit, but trap a length of electrical wire under one of the screw heads before tightening it down.
Refit the tank.
Finally, connect your new earth wire to one of the existing eart points near the rear light cluster.
Job done ...
(of course it's advisable to test the sender unit works before refitting the tank)
fix it .. bend it .. mend it
