Proform 3.6v Treadmill - seems to be sticking?

My treadmill is behaving very badly almost like the belt is sticking, It works fine when no weight is on it but as soon as you stand on it to walk or jog it goes fast, slow, slower, fast again and continues like that. It feels like im going to be spat off. Impossible to run on it and would like some advice on what could be wrong and how much its likely to cost to fix.