how can i repair the treadmill?

My proform treadmill has been used for about 1 year. We normally get on it and begin walking at 2.5 mph then gradulally upgrade to 3.8 mph. Workout period is about 3 hours at a session. Lately the treadmill functions in a jerky or choppy fashion. When we start it up and are not on it, it runs smooth, when we get on it, we can almost stop the belt and thereafter the belt does not run smoothly but in a jerky pattern. I have tightened the belt several turns but either tighter or looser the belt still jerks when weight is applied. What can I do.