Belt slip is usually caused by dirt or grease on the clutch faces, or improperly maintained clutches. Try removing the belt and cleaning the faces on both clutches with brake-clean and a clean rag. Wash the belt in warm water with dish soap and allow to completely dry. If you haven't greased your clutches recently, apply 1 to 2 pumps of grease in the fitting on each clutch. Let us know if this doesn't solve your problem.
