Problem with temp gauge

Hi, I have found a odd issue with the temp gauge. As soon as the lights are turned on the temp goes up, turn them off and it goes back down. What electrical issue would cause this?

no sparky here either, but as MM said, process of elimination:
Power-disconnect existing power  and going straight back to the battery +, then try with lights on/off
Earth- If its not power fault, do same with earthing straight back to battery -
Finally, if its neither of the above, it must be sender wiring: reconnect power and earth, and use your long length of wire straight to the sender to test.