You can also try 'glamor' if you want to be sure that the acceleration method is not the problem.
Option "SwapBuffersWait" "true" sounds related. But you might just have to wait until glamor acceleration matures. Oh and about taking a screenshot while resizing... I think this is why one of the screenshot applets allows you to set a timer.
You can try making VertRefresh in xorg.conf have a narrower range. Also does a command-line program like xrandr work to set the refresh rate back to 120? If so, you can put that command in your .xinitrc to make it run every time.