start it up look under bonnet and look for any coolant coming out from any where whilest giving a few revs of the throttle bodie might be throwin water out under load ive had this happen to one of my cars and it turned out to be this a very tiny hole in a ipe that was throwin water out at pressure when the engines revs are running higher therefore the water pumps throwing the water round at more of a rate causing the pressure to allow it to leak and obviously during idea not doing so allowing the coolant to flow correctly etc. worth a try