This is similar to a problem I had with my 9500 GT graphics card. For a few months everything was fine, but then it started to lock up when watching flash video. At first, I was able to close out of the browser and regain control, but after a few more days it would lock up for about a minute and then the BSOD would come up. I ended up RMAing the card, and the replacement has been working fine for about a month now.
Do you have another graphics card that you could put in to test if it solves the problem?