Total linux n00b here. Just installed the 8.04 release candidate two days ago. Please forgive me if this question is done to death...
So far any video I try to play redlines the cpu, then starts stuttering and balking until it's unwatchable. So far I've tried some Xvid .avi files and a Quicktime .mov under using Totem, Mplayer, and VLC. I believe I've downloaded the special restricted codecs and plug-ins correctly. (As best I can tell, anyway.) The files usually start OK, look great for about five or 10 seconds, and then gasp to a standstill as the CPU usage goes up to 100%. The memory and swap files don't appear to be hit particularly hard according to the system monitor. Usually the sound continues playing while the video stutters to a halt.
I have the same problem with Firefox 3 Beta 5 and the various flash plugins (I've tried something called gnash and now I'm using the latest restricted Adobe Flash plugin from the package manager.) The difference here is that the sound stutters, too, while Firefox eats up the cpu. (I see there are other topics about firefox & flash, but I don't see them accompanied by video woes in general.)
I'm starting to wonder if its not the software but the hardware, or the video driver. The laptop I'm using (specs in the sig) has an old Radeon Mobility 7500, so I don't think it's eligible for the current ATI driver. Any thoughts or points in good directions would be nifty.