My long experience with several random sleep or random shutdown laptops is that this was invariably a logic board problem, Apple used to take care of it .... and although this was some years ago—the first aluminum 15" Powerbooks—I think the issue is the same. Not a self-fixit.
In fact, early on, Apple replaced two machines for free.
This used to be common knowledge. I'm not sure the "Geniuses" have any—but if a logic board replacement doesn't fix the problem, the machine should be replaced. There's nothing you could have done to have caused a problem at that level.