Couple of things to check. First, did you use an OEM belt? Aftermarket belts can run oversized. Second, have you double checked that the belt is routed properly? Third, are the belt guards installed properly? Did you check the condition of the idler pulleys? Worn pulleys can wobble and cause the belt to jump. Finally, are you sure the tensioner arm is moving freely through out its full travel area, keeping tension on the belt? Any of these or a combination can cause your problem.
