This seems to be a common problem. I replaced 4 bit adapters in 3 months, then I got the rock chuck from rjustice. No more problems, a more vibration, cooler shaft, and a lot less noise. Not to mention the carves are so much better. Search for rock chuck and as soon as you can afford it buy one. With the rate you are destroying bit adapters, it will pay for itself in about a year.
Link to the rock chuck
http://www.cw-parts.com/