Has anyone yet mentioned the crank position sensor? :(
That is what was wrong with mine. It would fail without any warning signs or symptoms at all, and apparently was never in failure mode long enough to set any codes. (-only a couple of seconds, -just long enough to kill the engine, usually, but not always, -would sometimes re-light on it'* own)
And while I had the crank pulley off, I replaced the cam sensor too.
(per the recommendation of a VERY EXPERIENCED GM shop forman I know)
So far so good