That's the same set-up I'm using. Do you get this problem often? Is it reproducible? Is there a particular thing that you are doing when this happens?
Only seen it once so far, at the time I was running my i386 install of firefox (I have 32/64 versions installed mainly for flash) and had a couple of flash pages up and running at the same time. I noticed that my browser, and evolution became unresponsive at the same time and I had to force quit them.
I'm trying to push my firefox/flash again to see if it barfs.