Another thought, I have had ringed engines that seem to lose a bit of compression before completly breaking in. Not sure why, maybe the ring rotates and needs reseating. Seems to happen if I only do a short break in on the stand. Seems to be running great and has good compression. Then I put it on a plane, it runs great for a while and then either dead sticks or is hard to start. The compression seems to be way off. When this happens the compression comes back after a few tanks of fuel, then the problem never returns.