MX5 engine temp

[swansea v6]

bit of an odd one this, the coolant temp guge on my 1992 1.6 mx5 sits around half way on the gauge while driving around town but when i get on the motorwy it shoots up to virtually the top......i have flushed the coolant and changed the thermostat but still the same..............

[PJLarge]

You probably need a new radiator - it's a known problem. I had the same problem and changed pretty much everything in the cooling system trying to find the fault. It was the radiator.
You could disconnect the fans and start the engine and allow the stat to open. Have a feel over the entire surface of the radiator and you'll probably find cool spots. If so, it's had it. Flushes are no good.
The trouble with the OEM radiator is it's so thin to keep the weight down it has little capacity so even a small blockage or scale build up will mean the whole system takes a beating.

Phil.