the problem is that the endlinks and bushings freeze up and the bar no longer floats, instead it acts as a 3rd and 4th spring that the dampers are not strong enough to handle
this is evident by a rocking/wagging/swaying motion going in a straight line after hitting a pothole, one wheel gets compressed (as does the bar) and since the bar is stuck in position it just works like a spring oscilllator...
ive been over this a million times in my head and on the ground staring at my suspension, and finally i just bought the poly endlinks and bushings, installed them with plenty of grease, and the problem basically disappeared. This was about a year ago.
Recently, the problem came back, i have swaying motion on the highway for no reason except going over bumps, so I pulled off my endlinks, cleaned them of rust, re-greased them, and the problem subsided. I still have to re-grease the bushings to completely rid the problem.