Constantly turn on and off monitor

Greetings everybody.
Recently, I met with an unusual problem.
Mac Mini Late 2012, SSD 128 GB, Mavericks OS , APC UPS and NEC MultiSync EA 273WM 27 "monitor .
The monitor is set to 15 minutes goes into sleep mode after 10 minutes appears on screen saver .
It happens that although the monitor goes into sleep mode , it automatically turns on, shows no signal again goes out and so constantly . Not at regular intervals.
If you move the sleep mode to, for example 20 minutes, then for a while everything is fine, but again after some time began to appear (4 , 5, 6 days). Otherwise, the computer and monitor operating normally.

It began to appear after security updates 2015- 006
Tips ?
Thanks in advance.