I am using the newest RC, but downgrading to a stable build doesn't fix the problem. When I am running a VM, it will suddenly stop working completely. When I then try to turn it off, Parallels freezes and the computer slows down. There is no way to quit Parallels (force quite, kill -9, etc). I am forced to do a hard shut down. This happens a lot. Sometimes it is while the guest OS is still loading. Sometimes it is after a few hours.
