- head gasket checked via leakdown test, appears to be fine.
No. There are so many other conditions the HG is under during running: Heat, Boost, Expansion etc etc that could cause it to leak.
Chances are that because its been running so hot so much the head could be warped and HG cooked. Have you put the head check chemical on it? I would do that before anything else. It will tell you if EX gasses are in the cooling system and if so, HG or head gone.
Does it heat really quickly? or does it slowly rise in temp?
Agreed need to sort the bleeder out.