300 tdi temperature gauge

hello there I'm having a few issues with my temperature reading not being normal it's been on the high side sometimes near the red but other times it's fine I've changed the thermostat just incase no leaks have pressure heat in the range of 40'c coming out the heaters all the time I'm pretty sure it's an earth fault but there's an issue I can't explain and am hoping for some help with.
So my defender has had a 300tdi conversion by Haywards revive beginning of last year not had any major issues its got an electric fan with a separate thermostat control for it. But since it's been for some chassis repairs or a lot lol it's been messing around.
So if I have the heater and lights on then the gauge goes up turn 1 off it goes back to normal I've checked the engine temp with a infrared temp gun and I only ever get 80-87'c off the pipes and the temperature sensors probe doesn't matter how I drive it I check the thermostat for the fan kicks in around 90-91'c all is good but the odd bit is if I sit in a que then it's fine for 5-10minutes the it just starts climbing its doesn't matter about the lights or the heater go check it no fan and the temp with the gun is still 80-87'c drive off down the road maybe a minute or two at 30 mph the gauge goes back to normal my question is could this be still caused bey the same issue or do I have 2 problems? So for the length thanks in advance

Bubble in the system loitering around the sender?

Temp rise in a queue sounds normal if you don't have a viscous fan fitted so I would leave that bit for the time being. Try putting a jump lead between the battery -ve terminal and the engine block if that cures the lights on/off issue then you are looking for a duff earth lead (battery to chassis or chassis to engine block). If that doesn't cure it then you probably need to pull the instrument panel and check the temp gauge wiring from the gauge to chassis.