My old chevy/olds 350 diesel did the same heater stall at low rpm just as the heads were going and the gaskets started leaking. Air would build up in the coolant and migrate to the heater at low rpm. Then circulate as soon as the rpm picked up and water pump put out more volume. There was also a oil like film floating on top of coolant. (It's actually combustion byproducts). NOT AN EASY FIX ITEM.

 

I have the same problem so I will just go off this old thread. What I have found is that I have two valves that work off the same vacuum line that are not working. One is the valve that opens up the water to flow thru the heater core and the other is the one that changes the air flow from the AC condenser to the heater core. What controls that vacuum line? Or, does someone have a vacuum line schematic. I tried to find a chilton or haynes manual, but I guess they dont make them. Any help???