A problem has developed on my laptop over the last week and is starting to piss me off now.

I can surf the net fine all day and is nice and quick but the moment i go to watch a streaming video clip from any website it plays for a few seconds then started to slow down,jerking then completely stops...this then virtually stops the laptop from doing anything for a minute or two..everything comes to a virtual standstill.
When i eventually get back to my homepage things start speeding up again and is normal in a min or 2 until i watch a video again where the whole thing starts over again.

This started a week ago and has got worse ever since.

Ive done a defrag,2 spyware checks,deleted all my temp internet files,cookies and a disc cleanup and has made no difference.

Anyone got any ideas........have been on about doing a RAM upgrade as things have been slowing up slightly over the past few months....could this be the prob do you think bearing in mind its been running fine for the year and a half ive had it.