New MBA--refurb less than 14 days into ownership. When working on the computer, occasionally (about 4 times now) if I let it sit idle (I assume long enough for the display to dim), the screen will go gray and freeze/not respond to keyboard commands. The keyboard backlight will stay on. I can manage to reset it after manipulating the power button a few times. I do not lose any work; OSX allows me to reopen the windows I had up before the freeze. Any idea what this could be? Advice? Thanks.