I had a somewhat similar problem to what you all are describing with my ATI All-in-Wonder Radeon 8500 128MB card. I was getting strange flickering and "swimming" the severity of which depended on the refresh rate being used (it seemed to be worst at 60Hz). No, I'm not talking about the standard flickering of a low refresh rate, because my monitor is an LCD flat panel.
After agonizing over this repeatedly and cursing ATI, a friend suggested trying a different VGA cable instead of the one supplied with my monitor. I swapped it out for a higher-quality (presumably better-shielded) cable, and my problems disappeared. So, it turns out the problem was not ATI after all, but rather just a crappy VGA cable.
I'm not sure if that fix will work for what you guys are experiencing, but it's worth a shot.