I've had this problem, too. I bought the new Surface Pro (i5, 8GB RAM, 256GB SSD) from the Microsoft store in the Westfield San Francisco Centre soon after it was released.
My Surface would randomly hibernate (screen went black, audio faded out), even when I was typing or using the trackpad. A few seconds after the screen went black, I was able to turn the Surface on again. The black and white windows logo would appear, and
then the lock screen (with the audio to which I was previously listening playing). After logging back in, my session was resumed and looked exactly how it did before the hibernation.
I reset my Surface, and after set up, checked for updates, installed them all, and restarted my computer. After that, the random hibernation was still occurring. I exchanged for a new Surface, but this one also has the hibernation issue.
I know every newly-released product is prone to a few bugs, but this one is especially frustrating, considering the price of this computer. I really like the new Surface, but if Microsoft ends up leaving its users in the dark, I'll be returning it. It's
just too expensive to be shutting itself off.
To end on a positive note, the employees at the Microsoft store were really helpful and super chill.