Quicktime reeeal slow


Hi I'm running OSX tiger with safari as my browser and quicktime has started slowing down heaps when loading clips online. It often only plays audio on things that should be video, and sometimes won't play things at all. Quicktime and mozilla are updated to the latest version, so any reasons why this might happen?
Are we talking about streams or downloadable videos? I.e. if you go to Apple's trailer pages and choose a trailer... That's one that gets downloaded and only played when enough data's on your computer already. Or a QuickTime stream like the keynotes by Steve Jobs? Then it's the fault of your network connection.