It sounds like the fuel gauge in my 760 Jim - all the work ethics of a 1970s coal miner!
First thing to do is to ascertain exactly what the gauge is doing in relation to engine temperature.
https://www.ebay.co.uk/itm/Temperatu...O/392187836370
If you invest in one of those, take the car for a 5-10 mile drive at a decent sort of speed, stop somewhere safe and leave the engine running, pop the bonnet and aim that at the thermostat housing (usually where the top hose joins the engine) and read the temperature, it should be in the high 80s, preferably about 88C give or take a few degrees. If it is reading 88C ish then by rights the temp gauge should be at the 12 o'clock position.
If not, there's a very high chance your thermostat needs renewing. You can also use that device to check for hot or cold spots on your radiator, wheel berings, brakes and so on and many uses around the home too - it's invaluable for balancing a central heating system for example.
One thought - nobody has decided to wind PTFE tape around the temp gauge sender/sensor threads have they? If so, that will cause problems, had that on mine (except it was the EFi sensor they'd done it to!
) and it causes the circuit to break, leaks round the sensor and so on.