imac display keeps shutting off

I have IMAC 27 inch mid 2011. The display keeps shutting off while in use. It shuts off automatically all the time. I have checked my screensaver setting and power setting. None of these setting is affecting the issue. I also checked the temp and it is compeletly normal (40*c). The screen is completely black no image at all. i don't think it is backlight issue.

I have brought to the apple store 3 times and they said that they run the system for 3 day and they were not able to reproduce the issue. I brought the imac back to home and it happens to me after 30 mins. Apple store said they cannot replace anythig if they can't reproduce the issue. Then i brought my machine to the third party service provider and they also was not able to detect the issue.

Please help.. i am still deseparately looking for the answer