I have proform 350 treadmill (used) for the past 3 months. Everything was working perfectly fine until yesterday the belt start slipping.It works fine, if I don't stand on it.Sooner I step on it, it start having that jerk motion. My weight is under 200lbs. So I know weight is not the problem. Can somebody help me

The first thing that I would recommend doing is checking the warranty on it, and then, if the warranty covers what is happening, within a specific time frame in order to it to be covered by the warranty, would be to call the store where you bought it to see if someone could come out to repair the problem.

Hopefully, others will also come along to address your question.

Try this, about the middle of the page:
ProForm Customer Service: Frequently Asked Questions : Treadmill Troubleshooting

Thanks for the help.