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

If you have a heat gun,, I would do as Del suggests back at post #4, that being accurately compare temps at the 2 heater hoses and the top rad hose immediately after the t/stat opens.. I suspect that 1 h/hose and the top rad hose will be similar,, with the heater hose going into the water pump will be much slower to heat, indicating partial/blockage..

I believe 150* water thru the heater would still give considerable heat.. jmo