But the "halt" occurs even if VirtualBox isn't running, right? I assume that you can run your host OS without running VB. (I've never played with it before -- it looks intriguing.)

I still can't help but wonder if the problem would still occur if X was killed and you tested for the problem at the console. Just using one of the VC's would leave X running and not really tell you anything. If the problem disappears along with X, it might be worth upgrading/downgrading/renewing X.