Intermittent 100% CPU on all VMs

After over a year of waiting so as to prove the solution as valid, I'm finally able to post this answer.

Dell's default BIOS settings have C-States enabled, which puts the computer in low-power mode during idle times. This is what causes the VMs to spiral into 100% CPU usage on a Hypervisor host (VMWare, Citrix included).

The solution is to set the System Profile setting in the BIOS to Performance, as opposed to Performance per watt [OS] or Performance per watt [DAPC] (the latter being the default).

The relevant Dell documentation, pp3:

http://en.community.dell.com/techcenter/extras/m/white_papers/20161975/download

And this reply from one of the few Dell support engineers who's familiar with the issue:

The short version is: C-States disable additional processor cores during idling times. For VMs that are tied to a core (this is OS controlled, I do not believe it's configurable), this could result in them locking up, as they're attemping to perform actions with resources that no longer exist in their eyes.

Generally speaking, C-States are generally used on items like backup servers, secondary role servers (Backup dns, dhcp, Domain controllers, etc) so that way the backup servers can remain on, but in a low power mode to save energy.

Addtional Documentation can be found here:

http://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface

In a nutshell, power idling on a Dell server should always be turned off (set to Performance) for Hypervisor hosts.

Thanks to Eddy Simons at Kitsap Bank for helping me to find this solution.