Yep once your engine gets warmed up ....... I have seen that before..... Head gasket and a good valve lapping....
Once you lap the valves you will have to adjust the valve lash...... You will need to take the head off then the side plate to expose the valves ...... Then take them out and adjust accordingly.......
Now 1 other thing, you might not need to lap the valves... But you do need to check the clearance of them... If they are in spec then you are in great shape........
Someone else will chime in and either agree with me or give another diagnosis..... So just hold on a bit and see.....
Let us know Mon Ami ~!~!