Hi,
My .02 worth. Sounds like it might be a fuel delivery problem to me. How long has it been since you changed your fuel filter? Possibly when the temperature goes below freezing, moisture which has condensed in the fuel line freezes and blocks it just enough to cause your "hard stumbles", then when it clears or whatever, it starts. Do you notice the hard stumbles after you've driven just a short way, shut it off for a bit then try to restart it? I assume that once it starts it runs fine? The times it doesn't start at all could be due to complete blockage of the fuel line somewhere. Then it won't start till the temp goes up enough to begin to thaw it. Just some ideas I had from personal experiences. Hope it helps.
