Sometimes kvm has trouble running on some cpus. Unfortunately you end up finding this out the hard way when proxmox crashes and / or shuts down. As it turns out its our friend cpu c states.
The solution to the problem is limiting the c states to C1. We will do this by telling the linux kernel the maximum c state to use.
Edit /etc/default/grub and add the following line.
Next update grub.
Now reboot the system.
Your system should now be crash free.