can you check compression?
I had an engine where it mysteriously was cracking one of the insulators I think cyl 3 near the electrode of the plug. a few years later it blew a head gasket, I suspected later that the plug was maybe getting wet. it was whiter than the others. next time you pull the plugs check the electrode to see if it has a hairline crack in it. I think maybe it was sparking through eh insulator crack and not through the plug electrode. I did not at that time see head gasket failing symptoms. they were bosche plugs and I had it happen maybe 3 times.