Apple has released Mac Mini EFI Firmware Update 1.7 to address HDMI video flicker issues.
About Mac Mini EFI Firmware Update 1.7
This update 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. Do not disturb or shut off the power on your Mac mini during this update.
You can get the update from Software Update or at the link below...