strange temp gauge overheating problem
My GT jtdm has just started with an unusual problem. The temp gauge rises from 90 to 100 slowly then drops back to 90 slowly. This only happens at motorway speed 70mph up. It never drops below 90 and is rock solid at 90 when driving around town. At first I thought thermostat but after reading on here it looks unlikely as they usually fail open not letting the car get up to temp. Could be blocked radiator or water pump on the way out, but the most likely after lots of reading on here seems to be a failed first stage of the fan. This would hopefully be preferable and also the cheapest route to a fix. So I need to fix the fan. The second stage definitely works I can hear it kick in but I cannot get the first stage to kick in even standing still and revving the engine once warm.It just stays at 90.
So my question is what do I need to do to test the fan, or change in the way of relays etc.