Might be a burnt valve if it only does it under load. Easy test…….Get it to operating temp, and shut it down. Take the coil wire off so it won’t start up. Spin it over and pay close attention to the sound. Does it skip or speed up as it makes a round or do all of the cylinders sound consistent? If you have a skip, then check cranking compression to see which one. If it isn’t up top, it is probably an exhaust valve leaking.