What type of storage are you using, and what type/speed of connectivity are you using? You might be causing high disk latency in the guest OS such that it is unable to access its drives, which can cause the system to just stop responding. If that is happening, you might see some disk errors or controller errors in the system event log of the guest.
Also, if you are using software iSCSI on the same uplinks that your VMs are using, check the utilization of those nics and see if you're saturating them.