What is Causing my 7.5 Tonne 2002 Leyland Daf to Randomly Overheat?

I know nothing of motor vehicles so i am not qualified to offer any help or advice - a better person than me should come along and explain the likely cause...

If the over temperature reading suddenly appears and just as suddenly returns to normal then it's very likely that the temperature instrument is telling lies - engines do not warm up and cool down so rapidly - there's a lot of latent heat in a hot engine - this 20 second return to normal suggests a fault in the measurement and display system rather than with the temperature of the engine itself.

http://en.wikipedia.org/wiki/Engine_coolant_temperature_sensor

Good luck...

Clarkson
November 2014