When you say "freeze" which of the following do you mean?
1) The page appears to stall partway through loading, but you can still scroll up and down and click on links.
2) The entire browser stops doing anything, and you get a spinning beachball during the stall.
3) The entire system seems to stop responding, such that if you click to another application (such as Mail or iTunes) the computer doesn't immediately respond (versus the web browser being slow or stalling, but other programs being completely responsive at the same time).
Also, do you see stalls anywhere outside of Firefox and Chrome? Other applications? And what happens if you try the same pages in Safari? Same behavior, or something different?