Porpoising is speed and trim angle related. If you are porpoising at any given speed you have 2 choices: Either speed up or tuck it in till it quits. If you are tucked all the way in or going as fast as you can/desire, then as the other guys said, time to add some stern lifting; a non-raked prop can help with that if you are running a raked prop. (If you are unfamiliar with the term "rake", look in the prop sales section herein for a definition.)
On your prop cavitation, is it cavitation burn as ziggy indicated in his pics, or are you talking about ventilation....picking up surface air in the propwash. If really cavitation burn, what is the condition of your prop? If rough edges or deformation of the blade, it could be due to the damage, not caused by running near the surface. I ran that way for years, on a stock rig, nothing fancy, and ran the prop so high that you could hear the engine rpms and the prop blades beating the water's surface and never had any C burn. On this problem, if ventilation, tucking in will reduce/stop that too. If you choose to run high like that then "cupping" of the trailing edge of the blades helps the prop to grab....see definitions mentioned earlier on this condition also. With your mentioning engine rpm increasing when the problem occurs, that's "ventilation".
