I've been getting random shutdowns on my MBA 2013. It almost always happens when I've closed the lid to put it to sleep. After that when I open it the screen always stays black and nothing happens. I have to press down on the power button for a while to get it to reboot. I've noticed it happening since I upgraded to Mavericks. I thought it might be a problem because I just updated so I did a clean install last week but it's continued to happen. Any ideas what this could be due to?