Your "normal driving" are the "tests" I'm referring to. You're knowingly driving the car with (what could be) very serious problems and you're using the symptoms of said problems to troubleshoot. That's not a good idea.

The fact that it's less of a problem when it's cold only reinforces my thoughts on the issue. I bet your heater isn't blowing hot at all in this cold, is it?

Don't make the same mistake the kid did (that ignored my advice). Flush the cooling system the way I instructed and - at the very least - you should be blowing super hot air - even in freezing temperatures. I'm guessing it will fix your overheating problems too. Flushing the system is too cheap and too easy NOT to do.......especially with the amount of damage you can (potentially) do to your car by neglecting it. Don't wait for things to get worse.

Again: G/L