Check to see if the speed changes with nobody walking on the belt. If it runs normally with nobody on it then you could have a friction problem between the belt and the board. Use the tips in the image below to make sure that the belt is properly aligned with the proper tension. You may need to lubricate the belt. The part number for the lubricant for this model is 219168. You can order this lubricant from the
If the belt hesitates with nobody walking on it you could have a motor/motor controller problem or failed roller bearings. A technician would normally need to diagnose and repair this type of problem.