The belt slowing down can have multiple reasons such as a bad motor controller, the controller may need calibrating or lubricating.
I'd recommend you check if the belt needs lubricating first. To do this you first set the elevation to the highest setting. Don't turn the motor on and stand near the front of the walking belt. While standing near the front of the belt give yourself a firm push to the rear.
If the belt is properly lubricated the belt will take you to the rear edge of the treadmill and dump you off on the floor. If you move and stop then belt needs lubricating.
There is a lubricating kit to fix this problem. The part number for the belt lubrication kit is 219168. You can buy it at the following link:
. The cost is around $4.00.
I hope this information helps to resolve this problem but submit another question with any additional details for further assistance.