Well I changed the thermostat this afternoon anyway, probably the original one. I had an identical problem on my last truck and it turned out to be a bubble in the heater core, tomorrow I'll try burping the system again, I did it yesterday but didn't have much of an incline to work with. If that isn't it have to start messing with the heater core. Out of curiosity anyone know exactly what causes the heater cores to fail, just fouling from gunk or is it the material of the exchanger itself that degrades?

86 4runner, pro comp AAL, Kayline Soft top, 31's but all stock