pro-form 745CS?

The treadmill shuts off randomly. I can be on it one day for an hour and have no problem, then the next day it will shut off two or three times during my hour walk. It comes back on after I pull the key out and wait a few seconds. It always starts again. It has been cleaned, the belt is centered and in good condition. Treadmill is only 3 years old. Please help.