My 05 is blowing cold air out of heater after it is warmed up. At an idle it blows hot but after you drive a few minutes it blows cold. Under accel it will also cool right off. Today driving home in the bitter cold, cruising on hiway, blowing ice cold I pulled over to check heater hoses. Ice Cold, yet temp gauge normal, degas normal no venting. HG's changed with studs, EGR cooler checked, new cooler, new injectors. Ongoing now for a couple years. No summer heating issues. Very Confused. Help. Could it be thermostat?