Can you help...........the belt on my treadmill keeps stopping??

We have a Proform 10.5qm treadmill and the belt keeps stopping during exercise. It will work initially for a minute or two then just cuts out, if you take out the key and then replace it starts all over again and then cuts out. We have lubricated the belt and adjusted the tension but that doesn't seem to work. Has anyone got any advice for us to repair it? Many thanks.