You have a broken encoder wheel.
Remove the black plastic cover (six phillip screws) at the head of the tread (power cord feed through it). On the right hand side you will notice a black plastic wheel with holes cut along the outside, this is the encoder wheel and gets brittle and cracks or becomes misaligned. If the wheel is not in too many pieces you can use epoxy to mend. This part is HARD to find new.
Spencer, August 2010
