Why is my MacBook falling asleep with "Clamshell Sleep" but lid is not closed
I don't know if this might be pertinent to your situation, but I once had a very similar problem with a 2009 MBP. The weird thing was when I picked it up and moved it, it would wake up. This went on for weeks until I finally figured it out; it would sometimes fall asleep when I put it on top of a (closed) white MacBook. It turns out both the MB and MBP sense the lid is closed via a magnet in the lid and a magnetic sensor near the front of the machine. When I laid it on top of the MB the MB's magnet triggered the sensor in the MBP making it think the lid was closed. So, it might be worth noticing if you've got any strongish magnets in the area around the front of the machine when it spontaneously takes a nap.