Can no longer view MKV files

[brou]

I used to be running iATKOS 10.5.5 on my XPS420, but I wanted to upgrade to 10.5.6. I could never get the upgrade to work with iATKOS, so I switched to iDeneb.

When I was running 10.5.5, I could easily watch 1080p .mkv files. But I can no longer watch them. I can hear the audio, but the picture only comes through all pixelated (about 1 frame a minute).

Any idea what would cause this? And is there a fix?

Video is nVidia 8800GT 512mb. Not sure what else you would need to know.