We had the bubbling issue with varying degrees of severity. In the worst case in the summer at full load it was almost disastrous, as the entire system got aerated and temps just kept climbing until it shut down or was unloaded. It was due to the cylinder head to PCC surface condition. We ran the Wartsila cleaning tool (with felt) on it, but the surface was still not clean (due to corrosion issues). We learned that we could use sandpaper on that tool, and we re-surfaced it. Afterward…no bubbles whatsoever! Did the bubbling get worse after some maintenance, like after pulling prechambers? We had actually expected the answer to be exhaust valve seals or head gaskets, but turned out to be neither. We have (5) 20V34SG engines, approx. 10,500 hrs in 7 yrs.