I am not getting any heat to the passenger compartment of my truck. . . The engine heats up like normal but then cools back down to about 100 to 125 Deg. (According the the temperature gauge). I have replaced the thermostat but that did nothing for the issue. When I was under the hood, I noticed a vacuum hose broken so I also replaced that. When I am driving around 45 miles an hour, I have nothing but cold air through the vents. Once I reach 55 to 60, the heat starts working, but as soon as I am off of the interstate hwy., and in stop and go traffic, the heat stops working again.

I can't find anything in the Haynes Manual about the vacuum lines or an issue like this. Any help would be greatly appreciated.