Very erratic temperature gauge

hi everyone, please forgive my stupidity but I can't find the answer I need anywhere.

My 1995 300tdi 110 has a few issues. Biggest of these right now is the temperature. Head gasket went last year, replaced gasket and put on a complete new cylinder head, all good. Today the temperature gauge went higher than normal, on a long run, then settled back to normal, then went REALLY high, and then settled back to normal. No changes in the way she was running, no steam, still getting nice hot air from the heater. Extremely weirdly, the lights from high to low beam seamed move the temperature gauge up or down, randomly. I checked under the bonnet, pipes were mostly cool, no bad smells or strange noises, expansion tank still with the same amount of antifreeze as when we set off... When we finally arrived home the gauge was reading pretty high again, as soon as we stopped (left the engine running) the gauge settled back to the middle as if she had done nothing wrong the whole trip.

Is this electrical? An earthing issue? Dodgy connection in the dash? Or something way more serious that I haven't/don't want to think about?

Thanks, in desperate need of your help

Simon

I've had a very similar issue on my 300tdi some time back.

Turned out to be a poor earth. Check and clean the main battery earth, the chassis and gearbox connection off the battery negative.

You'll probably find the chassis connection to be very poor. Ensure its clean metal and a good connection and go from there. Also check the wire is intact for the sender on the thermostat housing.

Let us know how you get on. Hope this helps

Said it before on here................... get an old school mechanical gauge [Durite] 30 quid or so and 100% reliable - I cannot believe how bad the OE gauges are.