iMac screen sometimes becomes black

[eLDeus]

Hi
I would be greatly appreciated if someone told me what can be wrong with my iMac 27 mid 2010. After replacing of supply board in service center it felt well about half a year, but recently it started to blackening the screen after Mac OS boot and turning it whenever it wants, sometime helps to push the power button to see the screen again.
I tried the following:
- replacing LED inverter board
- replacing power supply board
- using iMac as external monitor with MacBook through mini-DP cable
- putting display on another working iMac (the screen after showing Apple logo starts to blink quickly altering from white to gray tones in left and right sides of the screen)
- reinstalling Mac OS
- display matrix connectors soldering
But nothing has helped
Where in your opinion could hide the problem? Thank you so much for your help!
Best
P.S. Sorry for language mistakes!
P.P.S. video how it looks like:

 

[Fishrrman]

After trying all that, it might be time to "try" a new iMac! (sigh)

 

[eLDeus]

Just wondering if this is familiar problem or not and if it's issue with the motherboard or display?
Thanks.

 

[Willo34]

Might need on of these types of repairs.......might go for a new iMac in that case
https://macrepairnl.wordpress.com/2016/10/12/graphics-card-repair/