Recently my Suburban 6.2 Diesel has begun to over heat in cold weather.

When I start it up,the temp remains low,untill it quickly rises to danger hot.All the while my Heater will not blow Hot air.Then I pull over to cool it down,then afterwards the motor runs at normal temps

I have changed the Thermostat,drained flushed and refilled the Radiator with an antifreeze mix,there is visible water flow across the Radiator

I had a theory that perhaps some gunk had frozen in the Cooling system,thus forming a obstruction which caused the Engine temp to rise,after stopping the Motor the plug would melt and normal cooling would resume.

This was however not the case.

Thankyou in advance.