Man those 400s idle high. If it has never done it before and just started I'm inclined to think its clutch or belt-related. If it were mine I would pull the cover, then run it a bit faster to run the belt up the sheaves a bit to see if it stops at a higher RPM. If it does, I would then measure the gap between the belt and primary sheaves to ensure its correct..then remove to inspect the overriding clutch/bearing because if it does quit when the belt is off of it, it's likely that's the problem. They do fail pretty often.