I have a 97 1.8 standard mx5. Today was hot in the UK (probably about
85F). On a long motorway journey the temperature gauge was creeping up.
This was at a steady 75-80mph. Slowing to 65mph lead to the temperature
returning to normal. Coolant levels normal.

What's the most likely cause of this? My ideas so far:

Clogged system
Failing water pump
Bad thermostat

Any opinions most welcome