Just dealt with this. Belt would hold still when stepped on. Opened the cover and saw it was the drive belt slipping when under load. Loosened the bolts holding the motor, shifted it back with a pry bar as far as I could, and retightened. Checked my walking belt tension, lubricated, and I was back in business.
FWIW, don't try to fiddle with the drive belt when it's moving, even slowly. It has a tendency to grab and mangle fingers. Maybe you aren't dumb enough to need this advice, but clearly I could have used it about an hour ago.
Good luck!
Ken Crosson
January 2010