overheating at speed

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

In article .com>,
wrote:

> Clogged system
> Failing water pump
> Bad thermostat

Probably not the water pump. Assuming you have no coolant loss, check
the fan(s) as well. My similar symptoms were caused by an intermittant
fan motor, which had a dead spot on its commutator.

---
Lanny Chambers
'94C, St. Louis
http://www.hummingbirds.net/alignment.html