If it was air locked it would overheat regardless of speed, so I would rule that out straight away.
If it was mine I'd be thinking it's down to a lack of cooling from the viscous coupling or a restriction or blockage in circulation from a stuck thermostat, or a dodgy water pump.
Simple inexpensive things first though, rad cap and thermostat and take it from there

Old rad has gone now chaldon3. I used to live in Spain 123hotchef, back in sunny uk now, though i still go back a few times a year 👍🏻👍🏻👍🏻

Turns out it was the viscous fan, took it off, opened it up, hardly any oil in it, so cleaned and refilled, just driven back from Clacton...80miles, no temp gauge rises at all....i am a happy man 👍🏻

Job done and a cheap fix in the end .Good pointer for others 👍🏻

what oil did u fill it with @DarrenSpain I have seen people use RC damper oil

I used silicone diff oil 10000, i read you can use 6000-7000 aswell but apparantly the aussies use 10000 as its hotter over there and makes the clutch kick in a bit earlier, not so great for fuel economy but keeps the engine cool earlier so thats the main thing!!!
You can get the oil on ebay or most hobby shops that do RC cars.

Quick update for everyone, with the hot weather the other day the temp gauge rose again when going over 3000rpm, and dropped when under. Now the weather is back to normal the gauge has not risen when driven above 3000rpm. Very strange.

Poss water pump issue, or small blockage restricting when very warm?

are the rad fins all clear and not bent up?