HD .mkv filr with vorbis freezing

i have installed the all i one fix but, on 1080x720 .mkv files with vorbis I am getting freezing during playback over the dsm-330. other files in lesser resolution all play ok.

My cpu is running at 100% but i would have thought a dual core pentium 4, 3.6 Ghz with 2 GB of RAM would be more than enough to recode it ?? Any ideas on what I can do, I am running the latest firmware (1.1.3.4) and server (1.5.0. build 75) and my radeon x1650 video drivers are all up to date as well.

This is driving me mad !!