This is a shot in the dark but some nasty hibernation bugs are fixed in 4.8[1]. It's probably worth booting a kernel from that series to see if it helps.
So far it looks like this may have been the issue. I've done some stress testing of things that always made it fail to resume in the past, and it has successfully woken up each time.
Thanks for the tip!
Great! Thanks for taking the time to reply. Unresolved questions always bug me.
Thanks for the tip. Install 4.8 now to try it out.