I am having the same issue but when i as a say"burb" the system i leave the fill cap on, the heat on high and remove the bleed screw completely, i also bring the RPMs up at the throttle body while doing this. I usually get heat back for a few. then it goes out again. I have to burb the system one to two times a week. it also goes thew a bottle of coolent a week. head gasket is good the cooling system is all new including water pump. the only components that are not new is the exspantion tank and heater core. but sence the heat some times works I figure the core is good.
sorry for hijacking your thred I am at a lose on this too.