The cooling system probably needs to be bled correctly, it is quite difficult to get all the air out. There are a few threads on the subject, here and in the Stagea section for M35's
If it continues or reoccurs, perhaps there isn't enough anti boil coolant in the system, and the water is flash boiling in the head and around the bore. (these run 95-100 degrees on a cold day) Drain and fill with the recommended coolant percentages and re-bleed a few times once it's cold through the rear bleed point only. Obviously you would be checking the thermostats operation while the coolant is dropped, by testing it in a pot of water on the boil, and replacing the radiator cap, just in case.
If you still have issues, you could have a blown head gasket... I have seen it a few times.