Monitor goes black on mini tower 2011
My screen goes black and often I completely lose the signal and need to reboot. Other times my desktop goes in and out (full screen to small screen).
Try resetting the SMC and NVRAM/PRAM.
Intel-based Macs: Resetting the System Management Controller (SMC)
Next time a problem occurs not the time. Then open the Console app in Finder>Applications>Utilities and look for log entries at that time. Post suspicious ones here.