A rotozip was not designed to be a router, or even to make precise cuts, so that doesn't sound unreasonable.
Why do you need a 6" bit? You should always use the shortest possible bit, as the longer they are, the less rigid they are, and the more they magnify your runout. And you'll always have some runout. A laminate trimmer will be better, but i still wouldn't recommend using a 6" bit in one. If you want the least amount of runout, your best bet is to get a Bosch Colt laminate trimmer and a precision collet from http://www.precisebits.com/products/...h_Colt_Collets