Why does a web page suddenly disappear and is replaced with home page?

Sometimes I'll be working on a web page and it suddenly disappears and my home page is loaded instantly. The back and forward arrows are grey. It happens most commonly when I'm working the USA Today crossword puzzle but has happened with other pages as well. It always happens when I'm entering from the keyboard; not sure if it's always exactly when I hit a key. It is not, however, always the same key. It's been going on about 2-3 months. Very frustrating. Anything on that page is lost. (I'm usually more than half way done with the crossword puzzle.) No idea if it's related, but I've had more lock ups recently also. Usually cured by getting to finder and force quiting something but ocasionally I can not get out of the spinning ball and shut down by holding the "on" button.

Geoff