We played on ps3 with friend, and we killed Antoine. Then we saved guy that hind on roof of some shope, but forgot about his girl friend in Antoine's fridge. Then when their time is out, hey both died. Girl, and even guy who was saved and was inside safe house.
I think it was made intentionally - survivors who bounded with each other, will die in same time if each of them not will be saved.

About random dying... at least on ps3 version, my friend noticed that sometimes text appear and say "some survivor is dead" and i'm not saw this message, because i don't know. And sometimes nobody really died. One time message said - "Rebbeca is dead", in notepad we found only one Rebecca - is key character Rebecca Chambers, and she was located.

I dont know how about PC version (it very simmilar with ps3 version, i even saw PC screenshor of 2 TK's fused with each other in one spot but in different angles, this bug also on ps3)
And because bug that broke 2nd player game when 1nd player load checkpoint or save, i had no chance to play normally in pc version)

(sorry for bad english)

They die if not rescued after a certain time, like, if you haven't even met them, and it's been a few hours, they'll die, haven't had an issue so far, just started a new game with the GFWL save issues. As for Gordon dying, no idea, sounds like a glitch, please submit to the glitch thread.