You may need to lap the gasket surfaces to get them totally flat.
You will have to remove the both the head and cylinder to do it. I duct tape 300 grit wet/ dry sandpaper to an old 1/4" thick mirror I have. Use water to keep the sandpaper from getting loaded up with aluminum bits. Refresh the water often. Move the part in a circular motion, reversing direction often, or in a slow figure 8. Be carefull to press down squarely and true to the gasket surface. Do not rock either part at all, or you will end up with a surface that is not flat all around.
I just realised that I have read that some cylinders have a raised lip on them. If yours does, do not lap it. Only do the head.