Does it also stop suddenly when it is turning without anyone standing on it?
Ok, so the motor is able to turn continuously...
Have you tried adjusting the belt tension?
You should be able to lift the belt slightly with a finger. It should not slip when you are walking on it.
It should also be aligned straight, and not off to one side.
Around 1-2 inches.
Ok, that sounds about right.
Approximately how old is the machine?
Does the underside of the belt appear frayed or worn?
Has the belt ever been replaced?
Ok, if the belt it not worn then the motor may be having difficulty when under a load - you can check around the motor for any signs of burning or carbon buildup which may indicate a problem, or you could have a service engineer come and check it for you.
Ok, let me check if the other techs have any idea on this...