How can I fix my Sportscraft TX400 treadmill?

I have an older Sportscraft TX400 treadmill that was given to me for free. Ever since I recieved it the treamill hasn't worked properly. You can somewhat walk on it but you can't run on it at all. It's like it boggs down when you are on it. The heavier the person the worse it gets. If you get off of it the treadmill works just fine. You can get off of it and turn the speed up as high as it can go and it will work perfectly, but once you get on it the treamill goes super slow or gets to the point it completely stops. I really want to fix this and don't want to throw it out if it's an easy fix!!! Does anyone have any ideas as to what is wrong with it!!!!??????