Ahh...I was assuming that you had a new one put on to fix the problem. My bad. If one has been installed it "may" have been bad out of the box. I would get another one and see.
Also Terry's suggestion about the wiring is a good one. It might be that a harness was nicked on the installation of the Engine.
It also might be if the car was in a bad accident that the Water pump cracked and later shattered as a result. You temp guage may not be correctly functioning to show the overheating. Let the car run and idle. Partially unscrew the coolant resivior to see if alot pressure bleeds off. DO NOT UNSCREW IT ALL THE WAY!!!
If there is alot of pressure your Water pump should be fine.
