S3221QS, backlight flicker

Two days ago, I unpacked Dell S3221QS and to my surprise, after start, it had really comfortable beautiful image. After a while however, I’ve noticed slight flickering in backlight. It is mainly visible when static picture like drawings are on screen. I was staring to white drawing and it is really annoying after some hours. Next hour I was so disturbed from it, I started to search what is behind, or what kind of help I can get. What was done already:

1. Installed new nvidia driver 456.71 for desktop nvidia GTX 760,
2. Monitor was set to factory defaults (reset was applied),
3. Turned off monitor and PC,
4. Monitor Self test feature check (STFC) was performed (no cable was connected except power cord). No mistake was found at RGB, or white screen within short time, but after white color was running 2-3 minutes, slight flickering was observed (slight change of white screen brightness). Maybe flickering in other colors is well visible too. But always needs to be run few minutes. Probably it needs to warm up a little.
5. I’ve tried to connect this monitor to other computer with Quadro P2000 equipped by latest drivers from nvidia. Monitor was set to factory defaults. Result was the same.
6. DP cable was changed to new one. Power cord was changed as well.

Monitor runs on: Desktop Nvidia GTX 760, Monitor Dell S3221QS – connected via DP cable (4K/60Hz – before DP, HDMI was tested as well, with the same result), windows 10 64-bit Pro, i7-2600, 16GB RAM.

No such issue was observed on any other monitors connected to those two PCs. Flickering issue can be observed on HDMI as well as DP cable. HDMI runs on 30Hz MAX. DP provides 60Hz. Monitor S3221QS has S/N: CN-01KW9C-TV100-07R-05VV-A00 ; firmware is M2C101

Do you have any advice, what can I try? I don't know if I should return this monitor to seller or whether I should keep trying to test. Next day it seemed, everything is OK almost two three hours, because I switched color from Standard to Color Space and I increased brightness to 85. When I did factory reset it is holding but after some time it appeared again but only for a couple of minutes. Thank you in advance to anyone who might give advice.