Odd that it would only happen sometimes.. what type of OS is on the phone.. iOS / Android? And which browser are you using?
Android, Chrome browser. On old phone it happened quite often. On the new one it happens not that often that I would become annoying, but still...
More importantly, does this happen only after you haven't visited in a while? Computer browsers freeze in the same way, but you don't notice it cause you think the page is still loading. Your phone is still loading the page (outside of a cache), but you think the page is already loaded because you see part of the page on your phone. You just can't scroll yet.
Hope that makes sense.
I get what you mean, but I think it's not the case. I left page open for a while and nothing has changed, I wasn't able to scroll up or down.
Though, on old phone I was getting message that Chrome stopped working, while now I didn't got anything. Then I just turned off browser and visited websited again, everything was fine.