If you have been torquing the head properly and regularly, then your problem is that the head or cylinder, or both, mating surfaces are uneven. Not an uncommon problem at all and very well documented. The suggestion made to you is spot on. Just need a piece of glass, a mirror, or even a good thick piece of acrylic (a true flat surface) to tape down the sandpaper to, and then work it in a figure 8 pattern while keeping it flat against the surface until you see a nice even wear pattern across the entire surface.
I do this to all engines that I build, regardless of how cheap or expensive they were, and every single one of them needed it.