Monitor Flickering and black out problem

I have mac mini ( mid 2011) running on

macOS Sierra version 10.12 Beta

Processor 2.3 GHz Intel Core i5

Memory 8 GB 1333 MHz DDR3

Intel HD Graphics 3000 512 MB.

So I recently bought LG 25" Ultrawide monitor with 21:9 aspect ratio ( Link for details: LG 25UM58-P: 25 Inch Class 21:9 UltraWide Monitor | LG USA ). It came with HDMI cable. I connected my mac mini to LG monitor through HDMI cable but it showed max resolution of 1920*1080 whereas my monitor has 2560*1080 resolution. So to correct this issue i bought new Active mini display port to HDMI with 4k resolution ( Link for details: ULTIMA Active Mini DP to HDMI Eyefinity Multi-Display: Amazon.in: Electronics ) which worked. But now the problem is my LG monitor blacks out for 2-5 sec and also sometime flickers while working. Somebody please help to resolve this issue.