Another good idea is to check for vaccum leaks, that get larger when is gets colder!

I have seen similar problems with earlier (88-99) cars. It turned out to be a bad (weak) coil pack. What we did (on advice from a GM tech) was to remove the coil and put it in the freezer . When you take the coil out, do a resistance check to each pack and if the resistance isn't the equal for all 3, there is your problem. These cars ran perfect once they started.

That'* a good test, I added that to the coil test write up. https://www.gmforum.com/t278818/

Try some gasline antifreeze . or HEAT as the americans call it. When you had the car in the garage for 2 weeks did they do all the test in the garage or leave it sit outside to get the car cold to do the tests.
If it sits in the garage all the time when they do the tests it will be like summer. when the car started good. I had a 87 Bonnie with a bad blockheater cord. We get temps of-40 F .The garage tested the cord in the shop after it sat inside most of the day said it was good . Took the car home next morning bairley started. took it back 3 times and last time I got fumming mad before they put a new cord on.
Plug your car in. duncan

Thank you. I'll have to check it again.

Thanks Duncan, I've tried that too....no result.

Thanks Dan. This sounds like a good idea. All looks well but car has 127,000 0n it.

Sarg

Ah I don't mind at all not having to plug my car in just so I can start it

I know this is an old thread, but in case it is the only one covering this problem, I decided to post my resolution.

After enough investigation, I figured out the problem! There is an air leak created between the throttle body and the main seal when it gets cold enough and the parts contract (I was parking on the street in Chicago winters, had the problem when below 20F degrees and windy). I replaced the throttle body seal with a new one and haven't had the problem since. It has been 3 winters with the new seal and not once has it happened. Hope this helps anybody out there with this problem. The seal is inexpensive, PN 12580919 / 405036 at Rock Auto: More Information for ACDELCO 405036