Vertical Stripes on iMac screen, please help me save him!

Hi all,

so my beloved iMac suddenly froze up (was running high demand video software) so I turned it off and then back on.

While booting it had now blue/green vertical stripes on the screen and no way of passing the login screen (it just stay stuck on a grey screen with no stripes)

All I could do was boot in safe mode, and once to the os it showed few less stripes but red.

So I started doing all I could think of: reset the SMC, clear the PRAM, single user permission repair and finally clean install the all thing.

Nothing did the trick, also diagnostics with Apple Hardware Test showed no trouble found.

Now being the impatient idiot that I am I took it all apart. Video card seems fine, logic board too, I mean I guess they look fine, not even sure what I was expected to find, maybe a burnt chip or something I don't know, I don't have that kind of expertise, maybe some of you do.

Also connecting a secondary display showed exactly the same symptoms as on the main screen, just with a different resolution.

Here are my questions to you, what do you think? Would you say it is a faulty logic board or a video card failure?

Please help me save my iMac, for some reasons I am really attached to this machine and really would love to restore it to its original glory.

Thank you all!