None of the above guys. If your motor is a good runner when it's warm, then in cold weather it is pretty likely to be pure tuning. If it's cold, you need to run it richer, as much as a half a turn, maybe more than you do in the summer. You do not need on board glow drivers, or foil around the engine. I have flown routinely at 10 below and less frequently as cold as 20 below. No serious problem, and no motor modifications either, except for mixture tuning. The big problem is cold air is dense air, and dense air means lean runs if you don't retune, but it also means one heck of a lot more horse power out of your motor.

One other problem. In cold weather, the motor goes from Hot to MUCH colder that usual, and this can work loose the screws that hold the motor together. Your problem sounds like a classic loose carb or back plate. Thermal expansion or contraction are allowing air into the system somewhere it's not supposed to go.

So, do the following:

1) Tighten down every screw you can find on the engine.
2) Retune your motor to run richer in the winter. Make sure you really have backed off from that screaming lean setting. Let it break into 4 cycle every now and then
3) Also, run LESS nitro in the winter. Nitro is a catalytic inhibiter, and if a cold plug is really your problem, adding nitro will make it worse.

I run 10 or 15% in the summer, and no more than 5% from halloween until about St. Patick's day.

Carburetor icing CAN happen, but usually our carbs are so hot that ice can't form, anyway, it usually only happens when it is really cold AND the temperature is close to the dew point. I've only been able to make an RC carb ice up twice and only in conditions when the full scale pilots were avoiding the air because of icing condition.

You can get impact icing, but if you do, a shutdown of the engine is good news, the carb iced up before the airframe did, and you got the plane down rather than spinning it in. I've had impact icing on carbs and airframes, but that's from flying in snow squalls.