Remove the screws and remove the front motor hood from the treadmill. Inspect the drive belt for looseness and wear. You probably need to tighten or replace the drive belt. I am sending you some instructions for checking the walking belt for lubrication. If it is getting dry it can contribute to the drive belt slipping.
To test the walking belt lubrication, do not turn the treadmill motor on. Set the elevation to the highest level. Stand near the front of the walking belt. Push yourself firmly toward the rear of the treadmill. 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, the belt needs lubricating.
The part number for the belt lubrication kit is 219168. You can buy it at [searspartsdirect.com
]. The cost is around $4.00. The application instructions should be in the package, but in case they are not here is how to apply the lubrication.
Turn the walking belt until the belt seam is along the top of the front roller. Raise the belt on one side in the center of the walking board front to back as high as you can. Apply Â½ the tube of lubrication to the walking board in a zig zag fashion moving from front to back. Move the belt until the seam comes up on top of the rear roller and lift the belt again and put the rest of the lubrication on the walking board in the same manner. Turn the treadmill on and walk on the belt for 10-20 minutes. Walk all over the belt to distribute the lubrication evenly on the walking belt and board.