iMac goes to sleep after start up

Hi,

I've got a 2011, 21.5" iMac. I've had a few screen problems with it, and it's had two replacements now.

The first problem was a screen burn, or some cloudy patch behind the screen, so I got the screen replaced at the Apple Store under the warranty. The new screen worked fine for about a week, then wouldn't turn on, so I took it back, and they replaced the screen again, as well as the baclight board. This seems to have (for now) solved the problem. It's been about a week since I got my iMac back, and it's screen is now going to sleep when it starts up. So, it boots up, with the Apple logo, then the screen shuts off and it goes to sleep. I have to press the power button to get it back on. I've also noticed this only happens when I boot it up at the start of the day, so if I were to restart it now, it won't do it.

I've tried resetting the PRAM and stuff, but it's not made a difference... Does anyone know what the problem could be, or has anyone had this issue?

Thanks