I just wanted to give an update for people still suffering with these issues.
I tried everything - monkeying with video settings, UPnP, upgrading Ubuntu to 12.04, downgrading and upgrading my Nvidia drivers.
At the same time I noticed that my CPU usage was flying incredibly high while idling, so just for the hell of it I edited my advancedsettings.xml as noted here:
http://forum.xbmc.org/showthread.php?tid...pid1055474
I've tested about 15 trailers without a hitch since then, so maybe my CPU was just getting overloaded? I'm a newb in that department, but that seems to have done the trick.
As far as my video/audio settings go, I have Vertical Blank Sync always enabled and Audio Output Device is Defaults(ALSA). Under Playback (Video), I have everything disabled but VDPAU acceleration and color conversion and Activate teletext.
Hope this helps someone.
EDIT: And I suppose I should point out that I'm on XBMC 11.10 now.
EDIT2: Aaaand, nevermind. Just did it again.