Streaming video kills Internet on MacBook

Hi everyone,

I've been experiencing a very annoying problem for couple of months.
When watching any streaming video (youtube etc.) on my MacBook [OS 10.6.4], after about 20 secs the video becomes slow and starts stuttering and glitching, then after another 30 seconds of such "horror", Internet stops working, and comes back only after shutting and re-opening Macbook.

This happens every time and in any browser (have tried Safari, Firefox, Chrome).

When listening to last.fm online radio or watching movies from my hard drive, nothing like this ever happens.

The computer is connected directly to a network cable, I don't use any wireless routers.
Internet speed is normal and is not the cause.
It seems that this problem first appeared soon after transition to Snow Leopard.

I have been desperately trying to find a solution or someone with a similar problem, but still no luck.

Do you have any idea what can be causing this?

I would be very very grateful for your help guys!