engine revs 4000 rpm/ 3 seconds on start up

[Long john]

Morning,
I have isolated the revving to the coolant temp switch on the head, when the car is cold it functions perfectly normal as soon as it heats even slightly the engine starts revving on start up, the hotter it gets the more it revs, for 3 seconds then back to normal. Unplugging this switch eliminates the problem, replaced it with a new one from the agents, no difference.
Does anyone have any ideas?
Much appreciated.