Audio / Vid looses synchronicity after some playback time

Hi Folks,

strangely, after some time i playback an HD file (matroska, 720P, DTS) the sound gets left behind.

I tried the VLC in comparison, which does not have this problem - but playback is much smoother with xbmc.

What can i do?
Thinking about switchin to win 7 actually, because it should have the better hardware acceleration support. Whatcha think?

Thanks for help people. This forum is great!
James

Hardware:

Windows XP SP3 MCE
Athlon XP 2400+
Asus A7n8x (nforce2)
ATI radeon 9500 pro (128mb, 250mhz)
Soundblaster Audigy --> SPDIF passthrough to external decoder