After disabling the VT-x extensions and several hours of work, and an hour idle, the VM is still running. I'm going to leave it up, but I think that the VT-x extensions, 64bit operational mode, or a combination/interaction of those factors are behind this problem. Hopefully it will be resolved soon as being unable to run in full 64-bit mode isn't helpful.
------ Next Day ------
Well, I left the VM running overnight with VT-x disabled, and it is still running without difficulty. Definitely seems to be a bug in the VT-x/64-bit operational mode.