so far i've found if i get a game locking up after a while, just stopping and either dropping to desktop with screwed colours or crashing on one image and needing reseting that its not enough voltage being used. if theres either type of artifacts your mem/gpu is probo pushed too far, poss not enough volts or too many or too hot. but plan crashing without artifacts i go back push voltage up a little more and clocks get stable. you should try at 1.3v and see if 625/650 becomes stable without lockups.