On most occasions were I have seen this, the lower board was bad.
Unplug the treadmill, then unplug the motor wires. Plug back in and turn on. If it stays on, the motor is bad. If it still shuts off, the lower board may be bad.
For continued support
http://www.mindbites.com/person/214-Mark
http://petersontreadmillrepair.forumco.com/default.asp
Mark Peterson, May 2008