[dumalkin]

Hi,
I have the following setup:
XBMC Dharma RC2, Windows 7 64 Bit Professional.
i7 based PC with NVidia G220 card.
This card has 3 output - DVI, VGA and HDMI. DVI and VGA can not be active simultaneously, HDMI can work as second output.
VGA is not used, DVI is connected to 24" LCD display (no audio), HDMI is connected to AVR input, Sansui 26" LCD is connected to AVR output.

When I turn on AVR it is detected as second display, I can specify dual monitor mode (use or not, mirror the same image on both displays or not etc).
When I specify to use AVR as the only display, and turn PC display off - everything is working OK.
If I do not use AVR at all - all is OK either (unfortunately no sound from AVR - can not find way to use AVR as sound device only).
If displays are in "extended mode" playback is OK as well (with all resolution combinations) - on both displays.

Problem arise when I specify mirror mode - both display show the same 1920x1080 desktop. It is not native (LCD display has 1920x1200 as native resolution, Sansui is HD ready with 1368x720 as native), but for my purpose - watching House when I am on elliptical - it is good enough.

XBMC UI is shown normally, but movies playback is jerky (CPU load is still very low. Hardware acceleration is allowed).

When I play the same movie on mirrored displays using VLC - playback is smooth.
So the problem exists only in the following combination:
XBMC is the player, both displays are active in mirror mode.

Any ideas ?

[TinkerDude]

I don't know if it'll help, but I had a similar issue playing movies on my Sony HDTV over HDMI. I wasn't using XBMC at the time, and the same problem occurred using Acer Arcade (The player that came with my laptop) and with PowerDVD10. While Googling the symptom, I came across a possible solution that sounded so unlikely that I initially discounted it. My configuration is similar to yours. I'm running an i7-based laptop with NVidia GTS250M video with HDMI out and Windows 7 x64 Ultimate. After reinstalling drivers and even rolling back to the driver supplied by Acer, all to no avail, I went back and decided to try the simple but unlikely suggestion. Voila! playback without so much as a blip. The solution: Change to the Windows Basic theme. Apparently, there is something about Windows Aero that does not get along with HDMI 100%.

May not be your issue, but it'll take all of 30 seconds to check.