External display occasionally turns black for 1 sec

Hello...

I have Macbook Pro (13-inch, Late 2011), 2.8 GHz i7, 8GB 1600MHz DDR3, Intel HD Graphics 3000 512 MB

and I bought external display Dell U2715H where is this problem:

1) If I connect Dell via mini display port to DP in (the cable arrived with the monitor), the resolution is OK (2560 x 1440), but dipslay occasionaly turn black for 1 second. Sometimes it is ok for couple minutes, but sometimes do not (3x turn black at 1 minute). I did not recognize what depends.

2) If I connect Dell via mini display port to HDMI, the resolution is lower (2048 x 1152) and I can not set 2560 x 1440 which is optimal resolution. In this case "turns black problem" does not happened.

I have OS X Yosemite 10.10.5 installed with latest actualizations.

I tried:

Kill all run applications.

Restart system.

Turn off and turn on macbook pro.

NVRAM reset.

SMC reset.

Can you help me please?

Thank you, Petr