Pages freezing up, constantly... why is this happening?

I'll be on a page, maybe Facebook, the NYTimes, Amazon, or really any site, and suddenly I find I can no longer scroll, or otherwise navigate the page. I usually have to refresh the page, not so great an option, if I was in the middle of doing something, and and then it'll work OK. I shouldn't have to be doing that, and wonder why pages will just freeze up like this.