Fan question!

[Lu-lu]

I have recently had my head done! (car not me!!) and a new fan fitted, since then occasionally when I start the car the fan comes on immediatly, not every time and not always for very long, yesterday it came on whilst I was driving the temp was normal, and it soon went off again, it came on as soon as i turned the key to warm the plugs up on my homeward trip having been stood all day! why would this be?
Lu ( a little paranoid regarding the cooling system)

[BUSHWHACKER]

The plug could be loose or the wire damaged, causing the fan to operate intermitantly. It's one of the wires that plug into one of the sensors on the thermostat housing just below the radiator cap.

To check which one is for the fan, (the other plug is for the temperature gauge) switch on ignition when engine is stone cold,(first thing in the morning is best) pull off one of the plugs. When you pull off the correct plug the fan will come on.
Make sure the plug is on securely and the wire isn't broken, if all OK then I suspect the sensor needs replacing or you may have an air lock in the cooling system.