I have a 1985 kc for Laser 1

I have a 1985 kc for laser 1.6lt automatic I put in a good second hand motor 6 months ago since put in I have snapped 4 fan belts. It has a new alternator water pump and I changed the crankshaft pully of the motor that blew up. When I put on a new belt rev the engine to approx 2000rpm the markings on the belt the line on it seems to be moving side to side. The deflection tension of the belt is the thickness of the belt
12mm approx. The only thing it could be is a warped water pump pully? Any suggestions would be much appreciated "o" when I put the new belt on drove 75kms got home and started squealing again HMMMMMM!