[PROBLEM] HD video - white noice

[aztech]

My PI have started to fail at playing MKV's.
Files are mostly 720p HDTV rips, but also 1080p h264 movies are failing.

Problem is that after a couple of minutes of playing, white noice start to appear.
The noice starts low (almost not visible), but after a few more minutes it takes over so that the whole picture gets white and sound dissapear.

I'v tried to fiddle with different OC settings, but no luck. Currently OC is at "none".
Watching SD divx is working just fine btw.

Xbian is the latest from today and the PI is a 128MB from one of the first batches.

This problem first revealed it self for me on beta 1, but a apt-get upgrade solved that. Now I'm on beta 2 and the problem is back. (Upgraded yesterday and did a full apt-get upgrade today).

Anyone have a clue to what might cause this?