This seems to be a problem with Airs...you're not the only one with the problem on these forums. It happens after the Air tries to enter its deep sleep, which will be after 1 hour -- this may explain why you're only seeing it occasionally. I used to have this problem, but its no longer a problem on my Air after I reset the SMC and reset the PRAM -- you can google around how to do both of these. Also, make sure your primary hard drive is selected as the Startup Disk in System Preferences. If this doesn't work, I suggest taking it in to let the genius bar look at it.