First question, is it a turbo or non turbo?
So youve figured our it is not electrical, and from what i have read it is deff related to the outside temp. And the car will idle at any temp, but as soon as you go to shift it into a gear it die's below the 70 degree mark. So im going to list a few things here if you have checked them and and posted back as so forgive me lot's of posts down there hard to remember all the info.
I live in Southern California so i feel your pain here. I know above 80 outside from a cold start mine start's like it's warm, if it's below that it does its normall putter start, putter putter putter vrommmmmmm. and i have everything etiher new or tuned properly.

First off i would look at your IAC motor, it is possile it is failing, pull it out and give it a quick cleaning and see if it help's. you can check the movement one you get it off with your finger and see if ti move's.

Second, take a look at your air to fuel misture, if it is off you could easly get results like this, Colder air is thinker then hotter air, maybe theres not enough fuel for the colder air and thus the mixture is not accurate. this is reaching on that one as it should run either way, but its possible.

Third, Look at all your gasket's, they expand when they get hot, it is also possible that below 70 you have a gasket somewhere on the engine shrinking just enough to let air in.

Forth, Have you done a compression check on the cyl's?

Fifth, if you havent done this check the o2 sensor, there is instruction in the F&Q on how to do this i believe, this could easly cause running problem's.

Off the top of my head those are the things i would check now, none of them will cost you anything but might get you somewhere with the problem.