Not always triggered within the browser. But nearly always when I trigger an action ... like for example clicking on a link in TweetDeck that launches a tab in either FF or Chrome. Initially I thought there was a conflict with Foobar because I nearly always have Foobar playing in the background, but since it has also happened when I didn't have Foobar running at all, that is obviously not the case.
But as I mentioned earlier, it happens maybe one or twice per week, so I still can use the PC most of the time. After backing up stuff, shall look inside to see if anything is loose first, as I have had that happen to me in the past; since the building I'm living in is under renovation, there is alot of drilling on the building walls and the vibrations might have loosened something.
Then I shall probably go for updating my video drivers (the ones I am using were installed when I built the PC 2 years ago, so that might also be a good bet).
Then probably the stress and memory tests ...