I'll just add that this post helped me - my symptoms, occasionally no heat, then more frequently no heat plus rising coolant temperatures. Finally rediculously high coolant temperatures and no heat. I replaced the thermostat (before reading anything!) with an aftermarket Murray thermostat, still the same problem. Reading through forums was worried about burping, head gaskets, water pumps, more than I wanted to do. But someone suggested a radiator cap. Again aftermarket Murray (that was easy on the way home) - that did the trick. I don't claim it is the solution for everyone, but it is certainly worth a $10 try.