Backlight randomly turning off

Hi, i have very annoying problem with my 27" late 2009 iMac (MB953LL/A). Basically the problem is that my screen backlight goes black from time to time (i still see content behind black screen with flashlight), it can be fine for a day and next day it can turn off 50 times per hour. After i put display to a sleep and wake it, it starts working fine again. I've read here that someone solved the problem by changing V-Sync cable, i ordered one, changed, but that didn't helped. The next thought was about overheating, so we cleaned everything from dust and removed heat sensor from screen and just sticked it on SSD, we also removed V-Sync cable completely. But the problem is still there. Besides, I had this problem on a completely cold computer that was waken up after night and with 100% running fans in a cold room.

Any further ideas or recommendations? Thanks.

yes i did smc and pram reset