99.999% sure it's a head gasket. Mine was also overheating in winter--same kind of issues. Hydrocarbon test of the coolant is a dead giveaway. I spent over 2 grand having mine replaced, but it came with the peace of mind that the shop stood by their work and if it developed issues again, I'd have that to fall back on. Temp gauge needle has been solid as a rock stuck smack-dab just to the left of middle since then.

If you hear bubbling inside the engine bay--something is wrong. Coolant only works when it's under pressure, and it will boil if it's not. if you've leak checked everything else, blown head-gasket is probably the culprit.