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.