Recently when you walk on the platform, the belt does these really quick hesitations.. also started noticing black fluffy stuff under and out the back of the platform. Thought it might be the drive belt.. Checked it and it looks fine. rollers are adjusted according to manual. Could it be the walking belt??? Rollers??? help!!! My son is having a fit he can't use it! THe treadmill is a Proform crosswalk 405E.