Videos last 2 seconds

Alright, I just got a new computer, and since I've gotten it, videos that I haven't downloaded (youtube, etc.,) will load, but only play 2 seconds before stopping. It doesn't say it was paused, and I can't make it start again. I can click later in the video, and it'll play for another two seconds, but that gets quite annoying quite fast. I've downloaded the DivX codecs, and I have flash player. I've tried uninstalling and reinstalling flash, and it makes a video or two work fine. But it doesn't work long. And I've done it a few times. Videos that I've downloaded work fine, though.

Any ideas?

 

Try downloading a youtube video using http://keepvid.com/ and watching it with an FLV player (I suggest Applian FLV Player, it's free... download link here). If you're able to download and watch the video with a standalone application without a problem, then at least you can rule out a few things.

A few things to try:
Try right clicking on the video, and choosing settings, and try making the cache higher.

Try clearing your browser's cache, exiting the browser, opening your browser again, then going to the video page again and try viewing it. A full cache affects performance.