Just curious. When shooting at high fps (5fps or more), does the action of the mirror flipping up and down very quickly contribute to camera shake? I've tried it my on 20D and I think it might, although just slightly. Has anyone used faster cameras and noticed this at all?
The high frame rate should not be causing noticeable problems because of the rapid shutter speeds you have to be using. Only on long-exposures does mirror movement genuinely cause problems. I'd suggest a review of your settings and camera holding technique to see whether the problem lies there. I frequently shoot 5fps with no problems with shake - what I sometimes do find is motion blur of the subject I'm taking.