Backlight randomly turning off

You have managed to bypass the protection logic! How about really fixing your system instead. Put back the thermal sensor and reconnect the V sync cable. All you have done is about the same thing as a biker riding down the road with one wheel missing... Sure you can do it but is it wise? Apple does a good job designing their systems and 99.999% of the systems don't require disconnecting things to work. In the long haul you're just wearing out your system sooner.

I did my research, i’ve found more than 6 threads when people with the same problem changed their led driver boards and it didn’t helped. I don’t see any problem of buying new board, i just think that problem isn’t here: failed led board turns off your backlight completely and forever, i don’t mind to fix this problem ‘normal way’, i just don’t understand how you’re so sure if there is clear evidence, that changing led driver board doesn’t solve this the problem. For example, a guy from my thread on apple forums: "I've got the same computer, and have had the same issues for over a year. Year before last Apple switched out my HD, installed a new logic board, new graphics card, and I have the issue. I have since switched out my LED driver board, as well as my v sync cable”. This guy changed his led driver board twice and you’re still offering me to throw $120USD into trashcan.

OK, lets say its not the LED driver board then what else could it be? The only other possibility here is the display unit its self. Now what are you going to do? If you want a better answer here you'll need to send it or drop it off at an Apple Authorized service center or Apple Store and let them diagnose it and fix it. Apple is likely to EoF this system this year so you may want to move forward in getting it fixed soon as you will loose the lower cost repair option (Apple Store).

Thats what i’m trying to figure out. What about cables? Might be failing LED screen itself, yes. The apple service will offer me the same stuff as for a guy from forums: Change your driver board, change your gpu (they even changed HDD (why?) etc. There is no problem with this, i just want to wear this computer till it dies and just buy a new one, instead of paying tons of money for new gpu’s, boards and led screen etc., especially when someone already did that and didn’t solved his problem. I had gpu failure in 2013 and i paid like 500USD, so you can imagine how much it will cost me to ‘find’ the problem in apple service. So in the end, i’m ready to buy new iMac, but as this still works, i don’t mind using it, thats why i’m using all kinds of diy solutions, which kinda works, but now i need to fix ‘blinking’ screen somehow.

While it's important to gain other opinions. One does need to filter a bit as sometimes the information is not correct or is someone's wishful thinking. When gaining help from people sight unseen you do need to validate there skills by reviewing other dialogs. Likewise, there is a limit here on what I can help you as I'm dependent on your eyes and what you document. So any errors can mislead. Which is why I think the best answer for you is to forgo fixing your own system here and let an expert fix it I.E. Apple Store.