I have the same problem. It seems to be an issue with the micro SD slot. When I remove the card and start up the phone, there is no problem. But, if the card is in when it starts up, the screen will timeout after 15 seconds, but turn right back on a couple seconds later. I've also noticed that problem persists even when an unmounted card is present in the slot, but as soon as I take the SD card out, the issue resolves. I've tried two different micro SD cards and the same thing happens with both, so it must be an issue with the actual slot.
Samsung has no idea how to fix this problem. I've sent my phone in to them twice and they have not fixed the problem at all.
I know this thread is very, very old, but I haven't seen this answer posted elsewhere, so hopefully this helps.