I'm new to wikis, but I'd like to share my experience with this issue. My frontend hardware:
Shuttle barebones system: AthlonXP 3000+ Barton core 1 GB DDR400? RAM geforce 4200 ti (128mb agp 8x) onboard sound nforce2 (Realtek) Mythbuntu 8.0 1280x1024@60 on 17inch LCD monitor
Using "High Quality" as a starting point, approximately every second in HD playback I would receive prebuffering messages. Normal playback was fine. Using Xvmc aggravated this problem, causing many prebuffering pauses to occur per second. This problem did not occur at all when playing back content that was not broadcast in HD. This problem was specific to this frontend on the network.
Things that had little to no effect: -fully enabling real time priority threads -sound buffering (or sound settings in general) -standard (ffmpeg) vs ligmpeg2 (since it is an amd system) -enabling always stream from backend
What ended up solving my problem: -increasing FSB from 100 to 133 -even better FSB from 133 to 166 -system unstable at FSB 200
I'd like to note that this is the 2nd frontend that I've set up on my network, the first frontend has significantly faster specs so I did not see this issue at all once the proprietary nvidia drivers were enabled.