Over the last month or so, I've noticed that if I leave my MacBook Air to sleep for over an hour (this is an approximate, I've not timed it) - It will shut down. So when opening up, I have to hit the power on. If I just leave it for a few minutes, it's instant on as usual. I've tried a couple of different reboots (resetting PRAM) but none seem to resolve this intermittent problem. Any ideas folks...?