I had a similar problem with my 2007 6.0 and i had replaced the alternator with one from auto zone and wound up taking it back to them and four alternators later I got one that lasted, they had a bad batch and it would work on the test machine just fine but once it was under stress of heat and actual working conditions it would fail. It has been fine now for over a year. knock on wood. but its a life time warranty so that is why i kept taking it back to them.