Re: 1969 25 hp evinrude sportster cavitating
Does the gearshift handle actually go to neutral when that happens? That would be the clutch dog. However, if it is merely slipping, probably the prop hub. You can tell by a simple test. Make a mark on the prop nut and another mark on the propeller, aligned with the first one. Run the motor till the suspected slippage occurs, then stop the motor and check your marks. If they no longer align, it slipped. If they do align, repeat the test because there is a slight chance it stopped slipping in the same spot.
EDIT: If it passes the hub test, it very well may be what you suggest---cavitation. Is the motor plenty deep in the water? That motor was a 20 year old design when built and was designed for wooden boats with a keel. The cavitation plate needs to be about an inch below the boat bottom. It's rare that you can get away with jacking it up like newer motors. Also, do you have floating weeds where you boat? Those things will make it cavitate in a heartbeat.
Somebody will come along and point out that it is ventilation, not cavitation. Hey, Ole Evinrude called it cavitation, that's good enough for me.