Ok its been a little while now, let me update on progress now.
I got new mushkin 1333mhz memory on Wed I think it was and let it run for 16 hours (!) to test, and it seemed to be fine. The computer has been booted since then without a hitch so far. Indeed, this seems to point all fingers toward that one module of g.skill. RMA time I suppose now.
Needless to say this was a difficult one to pinpoint, despite the fact I have been in the business for about 15 years now. What really threw me off is that both initially memtest86+
and Windows 7 memory diagnostic came up clean. This lead me down the wrong track for quite some time.
I'm going to consider this resolved for now, and if the machine stays up another week (I plan on leaving it running 24/7) I will consider it solid.
Now I have a spare GA-890GPA-UD3 without a cpu