Have you made sure that the lap speed is low? There are two significant issues with speed. Laps are often not flat and the variation in the vertical heigth cause vibration. So, the higher the speed the more the stone is being shocked. The higher speed will also push the cutting edges of the Diamond more aggressively in the horizontal plane and put stress on the stone that promotes chipping. Also, be sure that you have sufficient water carrying away the waste from cutting. A 1200 lap should be sufficiently smooth to never chip a stone at reasonable speeds.