We have the Profrom 835 QT Treadmill. The belt will move at most speeds, but will stop suddenly. It does not matter what the weight of the person iswho is using it. It does not matter what speed or incline yo are at. We cannot afford to get a new machine at this time. Can you help us troubleshoot the problem? Give a suggestion on what part might be the problem, prices, or where we might be able to purchase the part/parts? Thank you.