Very good point, I'll definitely be methodical. I'm fairly certain it's not a head gasket as there's no signs of it. I noticed there was a lot of built up corrosion in the housing, made me think it'll be the same throughout the cooling system. When I replaced the original thermostat housing I drained all the coolant and put it all back in, was thorough in making sure it was bled properly. Is there a flush I could run through it?