Whilst this topic is solved I just want to add: I must admit I am addicted to youtube (sad I know) and for about 6 months last year my internet was appalling due to high utilization in my area.
Luckily I found Speedbit Video Accelerator. I *think* it basically multi-threads the buffering of the video. It works on many other video sites too.
It's not the greatest application ever and I have had problems with it now and again where it didn't realize I was watching a video but those times were rare. If you aren't being capped and it's something else like maybe the youtube servers or a flakey line this might help. It's not voodoo it def works, and it works quite well.