'84 300d Turbo Will Not Cool Down
 

I've recently replaced my Aux. water pump and thermostat, replaced the gasket for the thermostat housing, run a liqui-moly radiator flush, and replaced my green coolant with Valvoline Zerex G05 diesel coolant, and yet my temperature needle will stay around 115 degrees C unless I drive between 30-45 mph, in which case it'll stay around 103. Yet, the coolant in my reservoir is cool. Does anyone know what might be going on?