David, to put it bluntly... your rider weight is not an issue. I rode with a racer pushing 268lbs+ and nearly as a fast as I am. Don't limit yourself out of the gate with that kind of thinking.
There is also a 80/90% fix for the bike moving around when you do to. Do you think making your saddle movements while on the gas vs on the brake will make any difference? Also, it has something to do with your issue above too.
EDIT: when pushing around 200lbs with a real good pace, 250's will flex/wobble and move around no matter what. It's just the nature of the bike at pace with weight. We aint 13yr olds that weigh 120lbs wet. lol