To be sure of what we're speaking of... The engine's model number would be helpful.
The 1979 100hp has a mechanical shift setup... capable of jumping out of gear, then being forced back in, which sounds and feels like one is hitting a underwater obstruction... BUT... having a internal problem resulting in "slipping" would be impossible unless perhaps the driveshaft splines stripped somehow. However, if that took place, the slipping would be permanent, not intermittent.
Has any work been done to the boat or engine recently... then shortly thereafter, this problem surfaced?
Possibly the rig (boat) has been trailered improperly, causing a hook in the keel that acts as a flap, raising the stern as the craft moves forward?
Or should the boat be stored at a marina (In The Water)... marine life may befouling the hull?
Has anything been added to the boat recently, modified in any way.... transducer, speedometer, something to the bottom of the hull or the transom that may be throwing a air pocket into the prop?
Is this craft new to you or a rig that you've owned for some time... and this problem just now popped up?