A bad bearing will shut the motor down just once. I'd go back to the fan belt. How old is it? It can be tight, but if glazed, will slip. Check pulleys for glazing. Try sanding the belt and pulleys. Does the alternator spin freely? I can't immediately think of why a slipping belt would cause the engine to stop suddenly (unless an old battery was really low after starting). So after it stops, see if it has enough juice to start again. If not, battery may be putting too high load on the alternator which causes belt to slip.