It happens a lot on Youtube and with some other sites as well. I was watching an MD vid and it happens a lot on that site. I have high speed DSL and my laptop is only a few years old. Any other help, thanks Kiwiol.
It's most likely a combination of things. Try doing a speed test and see what kind of download / upload speed you're getting and see how it compares with what your ISP says you should be getting. You need to do the speed test on a site that's local to where you are, to get an accurate result. For example, I'm in NZ and so, doing a speed test on a site here in NZ vs one overseas would give me a different result.
You can do standard stuff like check for spyware and use a good browser - Mozilla's Firefox is probably the best one out there, way better than ones like Internet Explorer. If you still have the buffering happening with Firefox and your speed results are fine, I'd just chalk it up to the YouTube servers having a bit more traffic than they can handle, which would slow down the loading time for an individual user.
Also, call your ISP's helpdesk - they would be the best people to talk to. Good luck.
