It happens with me too. Though only when I alos get a weird "loading" ad - the black bar with the white square on the one side and the red X in one corner and the red triangle on the other. Hit the red X to basicaly delete the ad off of the page. That ends up fixing it usually. It seems to be something with how the site interacts with that ad - when the ad attempts to reload it resets whether you're "clicked into" the text box or not.

At least that's what it seems to be and has been teh only foolproof solution I've seen.