Same issue here, but I don't think that it shuts down...I suggest that it's more like falling in a deep deep sleep. Everytime it happens, I have to force a restart (press the power button for 10-11sec.?) But sometimes it wakes up again, after holding the pwr button for 2-3 sec. Just tapping the botton doesn't help too often.
And everytime it happens, the WLAN is on. If i turn on the flightplane mode, it won't shutdown....pretty strange..