Intermittent Keyboard Failure when Waking from Sleep

So after a pretty catastrophic failure (see: Yosemite Upgrades: Lost and Broken User Accounts ) I have reinstalled Yosemite on my old tower-style Mac Pro and begin the work of restoring from backups.

A new strange behavior has cropped up - occasionally, when waking the computer from sleep and attempting to log in, the external USB keyboard (a Das Keyboard 4) won't work. There's no input in the password area, etc. The external USB mouse works just fine, and upon unplugging the keyboard and plugging it back in, the keyboard works again. There's no obvious failings of the keyboard, and I've tried several USB ports in case that was the problem, as well as resetting the SMC.

Any ideas what's going on?