720p movies studder sometimes?

my computer is a 3.0e socket 478 cpu, 2 gb ddr 400, and an HIS HD 4670 IceQ Native HDMI 1GB (128bit) DDR3 AGP video card. most of the 720p movies i watch work perfectly using VLC media player. some studder. the videos are usually in mkv format. i know my cpu is a bottle neck and my system is old but theres only a very few videos that studder. why? im running them off a seagate 2tb 5900 rpm sata hard drive with 32 mb cache. anything i can do to possibly downgrade the video a little bit? ive tried going to the task manager and setting vlc as high priority and it still studders.