Unexplicable extreme slowness on Dell PowerEdge R320, fixed only by cold reboot

At this customer's site, there are two new Dell PowerEdge R320 servers with the following configuration:

• A single 6-core CPU
• 16 GB of RAM
• 2×500 GB SATA disks in a RAID 1 array

The O.S. is Windows Server 2012 R2, used as a Domain Controller; all firmwares and drivers are up to date, and Windows is fully patched; the system load is usually very low.

All of a sudden, one of the servers slowed down to a crawl. And by "crawl", I mean "it wasn't even able to paint a window in a decent time". Doing anything at all, even right-clicking and showing up the contextual menu, even moving the cursor around, was an excruciating pain.

There was no unusual load on the server: CPU usage was 1-3%, RAM usage below 4 GB, no disk or network peaks, nothing at all.

There were also no errors whatsoever in any Windows event log (when we finally managed to open it), and the slowness didn't cease when the network cable was disconnected.

Rebooting Windows was useless, too: after a very long boot time, the system remained awfully slow as before.

Last but not least, there were no error messages either on the system's front panel display, or on the screen during the POST.

As a last resort, we decided to try a cold boot, and actually disconnected the power cables before restarting the server. This fixed the problem: the system booted normally and resumed full performance.

However, the question remains: WTF happened here?!?

And, more important: how can we make sure it won't happen again?