This just a comment more and nothing else, I've experience this issue with the first and 2nd generation of these machines and I was never able to really solve it. There has been a number of avenues explored and tried right from this website and still no "solid" solution found (the advise was sound but not a permanent fix. Now this is what I've been experiencing here for the last few days constantly with no "shutdowns:" Ever since I've attached an external device I experienced shutdowns, while powered up I've attached an external cooling pad (or anything utilizing the USB connections) and/or the power brick and moments later it shuts down. After one restart or two (while nothing was attached) it runs fine and without interruptions of power. So I've tried this method every time I used the Mac and in each instance I had no issues with random shutdowns. Try this, it's what has been (still is) working for me.