I have the same Problem with ac3 and divx as LaFounine, but I've found the possible reason for my problem: If I play a file with graphedit using the render file option, directsound filter is connected. with this the video plays fine, but I don't get 5.1. If I connect a WaveOut to the AC3 filter, and keep the directsound filter in the graph with its clock activated, I get 5.1 and smooth video :) If I activate the WaveOut clock the video problems are back. so it seems to be a timing problem of the audio-driver. I'm using on-board audio of my Shuttle FX41. The only problem that remains, now I can only play divx with 5.1 with graphedit :(