1974 15hp gears slipping

[Camoboat]

Hey everyone! I am new to this forum and an excited to get some expert help!!

I have a 1974 15hp Evinrude short shaft with slipping gears. When the motor is running and in forward gear, it runs just fine at idle speeds. Once I throttle up, the motor makes a 'clunk' noise and the speed falls off briefly, but will catch up and go to full speed again. This 'clunk' happens in 5-10 second intervals. I think the clunk noise is due to the motor slipping out of gear and then it catches again, making the noise.

If I hold pressure on the shift lever with my hand, the clunking doesn't happen. I have adjusted the shift linkage between the lower and upper units to ensure the linkage is correct. Because there was some play in the shift lever, I adjusted the shift linkage under the carb so there is no play in the lever when the motor is in forward gear.

None of this has worked, and the motor still 'clunks.' I am not sure what to try next. Any ideas of what to try?