Sounds like (from other posts) it's the rack. I'm 250 and ride with a heavy load on the back rack without any issues on a couple of different bikes (MTB, 3 speed, roadie). The rack itself does make a big difference. That being said, I know there are some builders who argue that a front mounted load is inherently more stable (CETMA and Kogswell come to mind) so I guess it's a matter of opinion.