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.
GRUB_CMDLINE_LINUX_DEFAULT="intel_idle.max_cstate=1 processor.max_cstate=1"
Next update grub.
> update-grub
Now reboot the system.
> reboot
Your system should now be crash free.