[Vwdiesel] the core of the Problem?

[Travis Gottschalk]

Remembering what others have said the heater core has really small passages and if you have used and stop leak type things in the system they can plug up on the cooling side of things, same if the water/coolant wasn't clean enough. As for the outside of the heater core all the 30+ years of dust, frenchfries, etc can settle down in there and the foam on the sides is likely gone as well and air takes the easiest path so air could be going around the core as well. After learning how big of a pain it is to get the heater core out (more so getting the clamps back on the heater core pipes) I just replaced both of mine. They are cheap enough when figuring you may have to go in again if it doesn't work the first time on something. I think around $100 and I got one from Carquest and another from advanced auto parts. Most of those things are made by almost the same companies anymore. Also a good radiator shop might be able to fix or tell you where to get  a new one. 
Travis G