I can help with advice to that issue. REPLACE HEAD GASKET. Once the seal has been compromised, you will never have a reliable seal again. It is best to lift
the head and to replace the gasket. The head and block surfaces will need to be clean. Torgue down the head and after another 5-10hrs, torgue it down again. This allows for take up, because the head expansion and contraction due to heat will compress the gasket further and stretch the bolts slightly. If the second torque down has not been done, somewhere down the track the gasket seal will fail just as you have had happen.
For general info, This re-torqueing should be done on any cast iron head/block, using conventional bolts.
Bolts that are tightend using the degrees of twist method are tightened once. They can never be re-used. They are discarded and new bolts used. This method is realtively common on more modern day engines with alloy heads. Some Aluminium heads use a special gasket surface that seals
to the metal surface and usually don't need retorqueing. HOWEVER, as there are many different methods, for all engines, it is essential that you read the manufactures specs and do and follow thier instructions and methods.