Th400 does not like a lot of rpm. They are more suited for a big block. Thee must be something miss aligned or put together wrong that he is just over looking. He will find the problem. If you don't turn it more than 6500, it will last forever. Anything over that, you are looking at a rebuild once a year. At least that's my experience, that's why I run a low rpm big block, I hate pulling trannys.