MacBook falls asleep

Hi folks, I am having the same problem as the above poster, and unfortunately the posted solution does not address it.

MacBook falls asleep while running

Starting out of the blue a couple of days ago, my relatively new Macbook Pro (running Yosemite) started randomly putting itself to "sleep" while I am working. By "sleep" I mean the screen goes black, it pauses iTunes, and it signs off of any Skype calls that might be in progress. I am mystified about why it's doing this or how to fix it.

Further clues to help would-be troubleshooters:

• This is unrelated to "Energy Saver" settings. My settings have it put the display to sleep after 5 minutes of inactivity, and the computer to sleep after 15 minutes. This "going to sleep" happens as I am working --literally while I am typing. It is not falling asleep due to inactivity; it's more like narcolepsy.
• Interestingly, it doesn't behave as it does during a normal sleep. In a normal sleep, you can wake it back up by tapping the trackpad, pressing a key, or pushing the Power button. When it does its narcoleptic thing, it only wakes back up when I tilt the whole machine to the right-- and this wakes it up immediately, each and every time.
• Thinking maybe it was a motherboard issue (though it really was just a blind guess), I took it in to my local "Genius Bar" today, The guy took it apart, said the motherboard looked great, and told me that it could probably be fixed by downloading the most recent Yosemite update. Unfortunately that did not solve it either.

Any ideas?