Hello. It could be a head gasket. If the gasket is broken in the coolant area the engine would run but would pressurise the cooling system. It could also be a crack in the block etc. I would do a cooling system pressure test. Pressurise the system to 15 PSI and ensure the pressure remains the same. What can be happenning is some of the combustion pressure is getting into the cooling system thus causing excessive coolant pressure. Sometimes this only happens after reaching opperating temperature in the case of a crack for example. If you remove the thermostast the coolant temperature should be lower. Just a thought. Regards Serge