Just a little tidbit of information regarding Xonar sound cards: The ‘GX’ mode, while I dread to think how deeply it hooks into DirectX (considering it crashes several old games), must be enabled for anything which involves certain clocks to not develop awful judder on the video side of things.
Basically if you use ReClock or play osu!, or anything of that sort (high fps/fast reaction games), enable GX mode. Without GX enabled, I’ve found there to be intermittent severe frame skip, which is only visible to the eye and not to any kind of software FPS or frame loss counter.
If you have a Xonar card and a high refresh rate monitor (it’s not very clear at 60hz), compare GX on and off with http://www.prad.de/en/monitore/testsoftware/pixperan.html.
If I remember correctly, with Aero enabled or Vsync on, the issue is not apparent.