I would say since you have tried another set of clocks, and the fuel reads OK with them, then your sender is ok and your clocks are not...or it could be the loom, as obviously you have to wiggle the loom about when switching clocks so could be a dirty/dodgey contact as much as it can be the clocks. You need to get your clocks and put them in someone elses, that will tell you if the fuel gauge fault is to do with the clocks for certain!
Temp senders and associated wiring is where you need to go next, a multimeter is your friend here, checking resistance of the sensors and the continuity of the wires coming to you clocks from the sensors/sendors. A:
Good luck :ILU: