What is difference from stock?

If nothing then something is fubar with what you have. A dial indicator on the pulley while you rotate it by hand will tell you a lot more than your eyeball will. Balancer pulley might be a bit tricky in the car but you ought to be able to do it. Don;t forget to closely check the brackets as well since they could have a crack and while not obvious under a light load could cause the issue.

The factory setup didn't really have a rep for throwing belts. So unless something is bent/broken, you deviated from stock or are spinning it to crazy rpm's it should suffice.