I have a 2011 MacBook Air (13", 4GB) that was rock solid with Lion for a year. However, ever since upgrading to Mountain Lion, I've been experiencing frequent freezes/lockups. Sometimes, my Menu bar will freeze, and I'll get the spinning beachball, then the system will completely freeze requiring me to power down. Other times, the machine will just instantly freeze, requiring a power down. Still, in another annoying situation, I'll close the lid on my laptop while plugged in, come back a few hours later, and the machine won't wake up. No screen activity, not even any light noticeable through the Apple logo on the back of the screen. NOTHING. It's getting to be to the point where I don't want to use my MBA anymore and try to do as much work as possible on my iPad. Nothing has changed software/hardware wise since the upgrade to Mountain Lion. I even tried disabling PowerNap, but that didn't work.