I have an old MacBook (6 years old) that I never upgraded from OS X 10.4 (I don’t think it’s supported by the latest updates anyway) so I decided to give it new life as my home media machine by installing Ubuntu 12.10, PMS and PMC. My media is on a NAS box that is available over wired network and mounted using NFS.
PMS works perfectly installed from the official packages, playing back media on my MacBook Air or iPad works perfectly, transcoding pushes the processor a little bit but I don’t notice this in the playback.
The problem comes when I try and play media with PMC on the Ubuntu machine. The playback stutters a little, like it’s dropping frames every now and then, whenever there are a lot of changes on screen. This happens often enough that it’s a little too annoying to watch. During playback the load average doesn’t go above 0.6 so it doesn’t appear to be a processor bound issues.
I’ve not done any configuration changes other than installing the packages necessary to install PMS + PMC on top of Ubuntu 12.10 desktop so I’m sure there are performance optimisations I could perform considering that this will never be used as a real desktop system, and I’m hoping that these optimisations will fix my little playback issue.
So my question is, does anyone have any tips for what I should look at tweaking first? Or what I should be measuring to see where the resource contention is as it clearly isn’t the processor?
I had the exact same problem until I got rid of the GUI running in the background. If you’re solely using it as a plex machine, I’d follow the steps I outlined at the bottom of the following post and see if that fixes your problem.
http://forums.plexapp.com/index.php/topic/49590-how-to-installing-plex-on-ubuntu-1210-64bit/page__view__findpost__p__326849
Thanks, for the next week or so I’m also using the same machine for ripping all of my existing DVDs but I will try this. I could have separate logins for ripping vs. PMC.
I also found the following blog post http://keepsmebusy.blogspot.no/2011/02/choppy-playback-xbmc-solved.html where he describes my symptoms exactly (problem most obvious during panning). I tried activating both the options mentioned there but as far as I could tell the problem just got a little worse, however I’m connecting via my Yamaha receiver so I was going to try connecting directly to the TV to see if that helps too.
I’ll try out both ideas tomorrow and report the results 
what video card do you have on that macbook? have you installed the official drivers from Nvidia or AMD?
I skipped over changing the lightdm greeter to the gtk implementation, so I should probably try that first, but this doesn't seem to have had any effect.
It appears to be an intel card
00:02.0 VGA compatible controller: Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller (rev 03)
Googling a little I see that there are guides for fixing problems with this card in Jaunty, but it's a little unclear to me if these are still relevant on 12.10
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