T620 with Server 2008R2 - Hyper-V Guest VM's have high CPU utilization

We have a new Dell T620 Server, running Server 2008R2. Server has 64 GB memory, and two 2.56GHz Intel E5-2640 CPU’s (6 cores each, so total of 12 cores). Hyper threading is enabled, so we see 24 cores in Task Manager.

There are eight SAS drives in a Raid 10 array. Each disk is 600GB, 15K RPM…nice and fast. Logically (in Windows) we have three partitions. The C drive is only used by the host server, and the guest VM’s are on the other two partitions.

We’re running four guest VM’s – all running Server 2008R2. Each VM is allocated 2 virtual processors, and anywhere between 8 and 12 GB memory.

Normally, performance on the host and the guest VM’s is screaming fast. However, about once a week, all of the guest VM’s will become incredibly slow, all at once. Looking at Task Manager on the guest VM’s shows their CPU utilization is around 100%. Sometimes the CPU stays at 100%, other time it bounces around, but is consistently very high.

In Task Manager, there is no particular process that is hogging the CPU – any process you run will take up a large chunk of CPU. Generic processes like “system idle” will often take up a large chunk. When you stop a service or process, some other one just grabs it, so the CPU stays near 100% regardless of what’s running.

On the host server, Task Manager shows CPU usage to be very  low, less than 10%. Performance of the host server seems to be as fast as usual, and unaffected by whatever is slowing the guest VM’s.

Restarting the guest VM’s does not help at all. As soon as they reboot, CPU is back to 100%. The problem occurs with all four of the guest VM’s, so there doesn’t appear to be a problematic guest.

The only “fix” for the problem is to reboot the host server. Once we do this, performance will be great for about another week, until the problem comes back.

There’s nothing in any of the event logs indicating what might cause the slowdown.

We've disabled and ever tried uninstalling the Antivirus, so that doesn't appear to be a possible cause.

If anybody has any suggestions, it would be greatly appreciated!