I've been having the same issue... but if you want to know what the cause is when it happens run "console" to see the error messages prior to the issue happening.
Applications -> Utilities -> Console
My issue turned out to be "disk0s2: I/O Error"
Macbook air was freezing at random times (5-15+mins), repeatedly . Only problem is after I rebooted (held down the power button for about 10secs) The error messages were not logged (probably because it is a disk issue).
Ran SMC, PRAM reset, disk utility and repair (hard drive came up clean); but still had issues afterwards.
Any who, I finally gave up and went to the genius bar. I have to wait for the SSD hard drive to come in, drop it off, then wait 3-5 days for it to be fixed. It was that or leave the laptop there and wait for about 1 week.
My airbook is < 3 months old... This isn't really what I expected buying an apple product, but I guess this happens with SSD hard drives? Only wish they would swap out the laptops like they do with iphones.
Anyone else with this issue?