Have you tried other movies you have outside your shared folder, eg: any you might have on disk or sample movies that were included with WMP.
Some people encode movies using very low quality settings or with poor quality encoders (also a small chance that the software used to encode doesn't give 100% compatibility with your media players on your platform-it's a playback issue after all. Also most net movies have been downward converted so there's a major loss of quality!) If you really think it's your dwnld software that's at fault, you could try something dramatic such as uninstalling it (trash the preferences) & reinstalling. If there's still no light at the end of the tunnel after that, then check your system with utility & virus diagnostic apps to check that your system files/apps don't have any corruptions. Actually, do the latter before attempting any uninstal. What is your OS version & when did you 1st notice this problem happening?