sure this is probably very 'off the wall', but I'll pass it on anyway. A couple of cars ago I had a 200tdi fitted to a Range Rover Classic. I had similar symptoms to yourself - run along at a steady 60 and it was fine, start to go a bit faster and the temp gauge would rise and go off the scale. However if I stopped and restarted the engine the temp guage would be at 'normal'.
I figured it was something electrical and as I was 'living with it' the altrnator went on the blink and I changed that and a 'voltage regulator' behind the dash - if memory serves. After that, no problems at all.
As I say - probably completely irrelevant, but you never know.
