Discovery 1 temp gauge

Hi all I wonder if someone can help me. If I go for a drive and get to operating temperature randomly my temp gauge will rise up to the red. Then goes up and down when I turn heater on and off or turn headlights on or basically just use any auxiliary electrics. All of the trucks lights also dim when the gauge rises at night. But as soon as I stop and let it tick over parked it’s fine. Slowly driving me insane

Best to ask over in the disco section of the forum.

Welcome to the forum

Main forum index:
https://www.landyzone.co.uk/forum

Discovery:
https://www.landyzone.co.uk/forum/land-rover-discovery.8

If it’s a early discovery upto 94 they have a resistor on the back of the instrument panel to keep the temp and fuel gauges accurate regardless of the battery state, so probably that, not sure what’s fitted on the later type,