I have a Mac mini (Late 2012)
El Capitan 10.11.3
Model Name: Mac Mini
Model Identifier: Macmini6,2
Processor Name: Intel Core i7
Processor Speed: 2.3 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 6 MB
Memory: 16 GB
Boot ROM Version: MM61.0106.B0A
SMC Version (system): 2.8f1
Intel HD Graphics 4000 1536 MB
Hard Disk To Sleep When Possible is (Un) checked
Screen goes black momentarily and comes back on it's own, sometimes puts a random color horizontal bar across the middle of the screen.
1. Started noticing this after bumping the OS to El Capitan
2. Tried 2 known working monitors
3. Tried 2 known working Mini Displayport (Mac Mini) to HDMI (Monitor) cables
Next I'll try the Mac Mini's HDMI Port to try to rule out the Mini Display Port.
To me, it appears to be either the graphics adapter, it's driver (maybe updated with OS X), or the OS X upgrade.
Up to this point, this machine and it's corresponding cable and monitor has been working for at least 2 years.