Fast broadband but slow streaming videos

Hello

I was wondering if any of you could help me solve my slow internet issue? I'll try and describe the problem as best I can but please let me know if I've missed any important details as I'm not good with computers.

I'm currently experiencing a slow internet speed with my Mac (G4 powerbook 12" Mac OS X 10.4.11) when connecting using wireless or an ethernet. I have just moved to a new flat and job so the internet providers are new but I don't think it is a problem with my broadband or router because it is slow at work and at home even though the download speed at both places is about 8mb and upload speed is over 500k. I also have plenty of free hardrive space (37GB).

It's pretty quick when I'm looking at pages without pictures, advertisements or videos but otherwise it can be slow uploading a page. Also when I want to watch a video on youtube or gametrailers for example, the video stutters and loads slowly, and the sound is out of sync even if I allow the video to fully download before playing it. However, if I watch quicktime videos on the apple site they load quickly and play perfectly. I've tried reinstalling all the video programs such as Quicktime, flipformac, adobe flash player and it hasn't helped.

This is very suprising because a few months a go it worked fine at my old workplace when I connected to the internet using ethernet. I think that it may be a problem with the graphics/video programs, a recent update or my internet settings but I really don't know where to start. It's quite an old computer but it has been very reliable up until now. I have a feeling that I'm missing something obvious and any advice anyone could give me would be greatly appreaciated.

Thanks