Sounds like you need to back flush your heater core. Disconnect the heater hoses at the firewall, use a water hose to force water in either of the heater core nipples and then do the same thing in the other nipple. Alternate back and forth from one nipple to the other until the water runs clear. Reconnect the hoses and see if that helps.

 

i went through the same crap with my '96 ram.what i did was this.made a vacumn reservoir out of a apple juice can and found another reservoir with built in back check valve.(not sure what vehicle it was from)installed them both,eliminated the vacumn switch for 4 wheel drive.plugged those lines off and added a manual cable for engagement.been 6 years now with no troubles.

 

It took me eight years but I found my problem. It was in fact a vacuum leak. Until recently the only symptoms were as I described above. Then cylinders seven and eight only started to foul out by oil deposits. After a quick check to make sure there wasn’t an oil gallery between the two cylinders getting past the head gasket and a quick check of seven & eight’s fuel injector’s resistance (just to be sure), I pulled the intake manifold and sure enough, the lower intake manifold gasket had failed and was sucking air and oil out of the crankcase into the manifold just under where seven and eight’s ports are located. They sucked up the oil and fumes screwing up my heater vacuum functions and fouling out the spark plugs. After a quick gasket replacement and proper torqueing…All is well again. Now I have lots of heat and no fouling. But I have something new I have to crawl under the vehicle and replace my upstream 02 sensor that is malfunctioning.

 

O2 failing is also a product of the blown plenum.