Apple has posted Mac mini EFI Firmware Update 1.7 (http://macte.ch/utbTb). It addresses HDMI video flicker issues on Mac mini (Late 2012) computers and is recommended for all users.
The Mac mini EFI Firmware Update will update the EFI firmware on your computer. Your computer’s power cord must be connected and plugged into a working power source. When your Mac mini restarts, a gray screen will appear with a status bar to indicate the progress of the update. It will take several minutes for the update to complete. Don’t disturb or shut off the power on your Mac mini during this update.