Pull the head back off and get some 220-grit wet/dry sandpaper and a very good true flat surface like a tempered glass panel. Lay the paper on the flat surface and hold it. Place the gasket surface of the head on the sandpaper and slowly scrub it around to sand the gasket surface. Look at the gasket surface and you will see the results of high and low spots.
Sand it until it is nice and level all across. The old head gasket should be fine unless it lost some face material, if so, replace it. Reinstall the head and torque the bolts down equally in three steps to spec. Button it back up and adjust everything and run it to see if problem is resolved.
The next possible issue is the crank case breather has failed. It is rare, but it can happen.
