Any fix for Macbook Air 2013 that goes black till restart?

Hi there,

We have 3 MBAs here in the office, all Mid-2013 models, all purchased at the same time, and they all experience the same issue. They all will randomly go to black screen where a hard reset is required to get video back. There is no rhyme of reason to it, as they'll do it while writing an email, or doing nothing at all. They've been updated to 10.9.1, and the issue persists. Reset PRAM, SMU, tried a fresh install of hte OS, still happens. This is frustrating as the systems are used for business, so if anyone has any advice on a fix, please, please let me know!