Happy New Year.
I guess the first thing is to run through the obvious:
Did you get the correct thermostat?
Have you flushed the system recently?
Do you have the correct amount of anti-freeze in?
What happens if you don't throttle back? Does the temperature continue
What is it like in heavy traffic - normally cars are fine at highway
speeds and only really heat up in heavy traffic?
If the fan is late in coming on whilst on the motorway but OK in
traffic, it might be worth checking the fan sensor to see if it is
being affected by ambient air temperature.
Mobile: 07971 540362
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift