[Herbie Null]

I had this problem once, mine was a aluminum head on steel block with a single layer head gasket. I chased the problem all season, water was not getting in oil or coming out of exhaust so I just left the pressure lever up on the radiator cap. When the engine was torn down the head had warped some, resurfaced head and used Cometic MLS gasket and never had anymore issues. I do believe the MLS gasket would have fixed it. I dont know if you have aluminum head are not, just passing along what I came across.