Why does my Proform 520x slip when I step on it?

I have a Proform 520x which recently began to slip when I stepped onto the walking belt. I took off the motor cover and set it to the lowest incline and 1.0 mph., and stepped on the walking belt. The motor runs and the flywheel moves but the motor belt and front roller/pulley as well as the walking belt I am standing on all stop moving. I've tried to adjust the rear roller...probably 3 or 4 full turns each per the manual but that doesn't seem to help. Any ideas? Is this treadmill worth repairing?