I'm thinking if there is a small crack in the exhaust port above the valve or maybe in the valve seat, it might not show up until the head heats up as in when it is running before the gasses find their way to the coolant passage?
Lol that sounds about right when it comes my car. Never the obvious symptom or remedy but some random, sporadic, mis-hap that makes me scratch my head for a month.
I upped the pressure even further on the radiator pressure tester and left it for a while and still no leakage. When this happened before it was at a cold start and only for 20 seconds or so. I have lot$ invested in these heads so praying it's not them.
