When you found out that the cylinder head was warped, did you have it machined or replaced? If it was me I would go as far as doing a leak down test as well just to cover all basis. I know a friend who had a compression tester that was giving wrong readings. He did a head gasket job and still had the same low compression readings. He tested it on another known good engine and it read that it had low compression levels. Needless to say the tester was bad. It may not be the case in your situation but just a thought.