Broken logic board?

Hi,

I bought my iMac 27" late 2013 (A1419) just over a year ago, no more warranty. I usually leave the iMac in sleep mode over night. This morning i found the screen totally corrupted with random horizontal lines. It was not possible to make out what was displayed so i restarted the machine. The screen was still corrupted after several cold starts and disconnected power. Eventually i did nvram reset (Alt-Cmd-R-P) and the displayed cleared and it booted up properly. However, i could still see some "imprinted streaks" in the center of the screen, about an inch wide from top to bottom. I then tried to reboot in repair mode and the screen again went totally corrupt. I repeated the nvram reset and the screen cleared and it booted up. After a few tries i managed to get it in repair mode with working display and did a disk check that indicated no errors. Now, after working with the machine for a few hours, the imprinted streaks are gone and everything seem to work, except that the screen get corrupted as soon as the screen goes to sleep. The only way to fix this is a reboot. I no longer need (so far) to do nvram reset to get the screen working.

Is this a broken logic board? Or something else? Any help most welcome!

Thanks in advance,

Mike