RE: mystery cooling problem
In my '73 Challenger, I did replace the thermostat that fixed the problem and it worked fine. What you described tells me that is what it is.
I would for sure flush the radiator. Many places will flush for you several times but you have to ask for it and make sure they do it
before they fill it back up with coolant. Otherwise, they will charge for the extra coolant. The path going to the heater tells me that possibly it has corrosion in it.
If you do those 2 things, I think you will fix both problems.
It helps to have a heater in the winter.
Best Wishes!
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