I had a very stable install of VirtualBox on Ubuntu 12.10. I recently upgraded to 13.04 and all of my VM's do not work any more. They boot, but after a few minutes they crash completely. I am at a loss as to how to begin diagnosing this. This is with both Windows and Linux guest VMs. I also tried making a new VM under 13.04 and that one crashes after a few minutes as well.