Display goes black randomly(not backlight issue)

It boots but 1-2 minutes later display goes black and it can be fixed only by getting iMac into sleep mode and waking it up. After waking up it works 1-2 minutes and goes black again. It's not a backlight trouble, screen seems like turned off. All diagnostic LED's are on. I've reseated all LCD cables but the issue remains. This iMac isn't from recalled batches.

What can it be and where can I get service manual for this device?