I think you have some easy fixes (possibly) suggested right here.
First, if you feel your original gunsmith is a trusted and experienced guy, I'd take it back to him and explain your issue...cold weather functioning. He could re-clean it and lube it with one of the "dry" products mentioned earlier, and I would think he'd probably do this for much less than the origianl charge since he worked on the gun and you still have an issue. I'd also ask him if he has any ideas.
Winchester may have records of your gun if it was repaired under recall. And if they don't have those records they may be able to confirm that your gun does require some work.
Those Win 100's are little beauties ... when they're working.

Refrowridge .... you are one lucky guy. My dream (one of 'em anyway) is to have both a Winchester 100 and a Winchester 88 BOTH in .284
What a sweet woods round. In my opinion the .284 was years ahead of it's time, and in stronger actions it is a surprising performer.