Subaru Problems? Ask a Mechanic for Answers ASAP
It sounds like you have excessive "back pressure", which can be caused from Exhaust entering the Cooling System from a head gasket leak near a Cylinder or bad water pump not circulating the coolant when the thermostat opens
You need to have a Reputable Radiator Shop perform a Exhaust Gas & Pressure Test.Be sure Radiator Cap is correct CapMany times a bad head gasket or crack will not show in the exhaust pipe & cause extreme back pressure
thank you for the "added info",yes could be gasket, as it takes time for exhaust pressure to build on a "small leak"I still would have a Reputable Radiator Shop perform a Exhaust Gas & Pressure Test.
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