Okay, so I thought for sure this was an issue of a dying GPU. I moved my secondary display (a Vizio TV) from the HDMI port on my GTX 460 to my GTX 670, and for a while the issue appeared to have been alleviated. The display is used for 24/7 XBMC. Periodically, the display will cut off and then back on, and sometimes display the "Not Supported" message for an undetermined amount of time. The TV works fine with other devices, and never cuts out otherwise. Could the problem be the HDMI cable I'm using? Its definitively not my GPU as I used the same HDMI port on a different display before and had zero issues with it.