Seems like a graphic card or graphic card driver issue to me but I'm guessing. Any problems with your monitor playing videos on-line? Is your hard drive full? Reinstall QT maybe? Is there program that can play without distortion?
Mac Pro, macbook pro, Imacs (i7); Camcorders: Panasonic AG-HPX170/AG-HPX250P, Canon HV30/40, Sony Z7U, VX2000, PD170; FCP 6 certified; write professionally for a variety of media; teach video production in L.A.