Even though the radiator's temperature doesn't vary much, that does not mean it's dissipating heat sufficiently. An evenly coated inside will still be a bad radiator.
You've not checked the flow from the pump. The impellars can become eroded and the pump not move fluid sufficiently.
Radiator hoses can collapse internally and restrict flow if the lining seperates fron the fiber-impregnated outside.
If all these check out okay, I'd want to do a pressure leakdown test on the cylinders even though you found no evidence of exhaust gasses in the coolant. Just to be sure.