Re: Random System Freezes in AMD64.
I encountered two types of hangs with the beta NVidia x86_64 driver. One of my regular hangs was fixed by disabling powerd from running. This definitely fixed a very repeatable kernel lock.
My other hang, like you, happened when I ran VirtualBox. I read somewhere that disabling Linux ABI compatibility in the NVidia driver solved this for someone. I re-installed the NVidia driver removing Linux ABI support ( see Chapter 5 in the NVIDIA-FreeBSD-x86_64-195.22/doc/README ). I ran VirtualBox for over an hour the other day with no hangs. Prior to removing Linux ABI, VirtualBox would hard lock the system within a few minutes, so I think that fixed my VirtualBox hang issue.
Worth a shot of you haven't tried either/both methods.