Whilst driving to work this morning, i had a large temperature symbol with a sop warning display on dashboard and temp was reading 120, whilst at 70mph. Just at that point hit road works so was unable to stop and dropped to 40mph and then the temp reverted to the normal reading of 90 degrees.

At the next opportunity to gain speed the temperature then started rising again (no warning symbols this time) but dropped back down to 90 degrees at 30-40MPH.

On arrival at work the electric fan had kicked in, there is no loss of coolant so i assumed this to be the thermostat. However in theory the
radiator fan should not of kicked off, because the coolant in the radiator would still be cool if it was the thermostat staying closed.

Anyone have any suggestions to this?