I would imagine you are having this problem because there is too much friction between your running deck and running belt. So when you don't stand on it there is no friction. To resolve this you will need Silicone Lubricant and apply roughly 10ml every 6 weeks or so to reduce the friction, this will also give your motor a longer life, as it will stop the power pushing back through the motor and burning it out.
jimmy, March 2015