Safari SUCKS. It bombs two or three times a day here. Usually ads crash it. When more than 4 tabs are going and one gets a complex Flash-type ad.... BOOM! Everything freezes and disappears. Annoying. Started for me with Snow Leoptard.
As with most people's complaints, this is Flash. Apple doesn't make Flash. Apple is trying to get rid of Flash for all these reasons.
I use ClickToFlash and all this Flash crap goes away. Try it.