remove the motor hood and see if the pulley on the front roller
is slipping. It normally makes a zing kind on sound.
If this is the case, when it slips, the walking belt and and the front roller will stop. The motor drive belt and roller pulley will continue to move.
You can reach me if you need more help
http://www.mindbites.com/person/214-Mark
Mark Peterson, April 2008