Can a lower unit slip??

[rhaser]

i have 76 85hp that runs like a top. after i get up on plane the motor revs like the prop is cavitating. i changed the prop same prob? as long as i have it under load it runs fine? i have changed the control cables? i don't think it's not engageing all the way becuase sometimes i have to go to reverse to get it back to nuetral. can it slip at high speed? this is the oldest 85 i've had and never seen this? any thoughts?

[boats.net]

No lower units do not slip they are gear to gear. It is possible that the motor is not far enough in the water causing it to cavitate.