Cooling issue, but this time it's to cold...

Well very first thing is to warm it up and "see what you have." As you yourself said, check with a thermometer that you figure is right.

FEEL the two heater hoses with the heater set up for "full on" heat. They should both be hot, and about the same temp. If one hose is somewhat warm but the other markedly cooler, then you have a coolant flow problem.

Don't discount anything.....pieces of trash caught in the nipples / connections / hoses, and certainly a plugged core.

I would but a new 190-195 Stant and put that in.