1977 omc momentary pops out/in to gear under load

I have a 1977 century with 235 omc electric shift. engine runs beautiful, drive and reverse seem to work perfectly as long as I keep it under 1500-2500 rpms, above that I get a split second "slip" or "pop" sensation in the boat. this will continue to happen randomly until rpms brought back down. yesterday I tryed to push through it, and see what happened. It slipped/popped a bunch of times within a few seconds and broke the shear pin and hub for the prop. I figured this was the original problem, and replaced the prop, pin, and nose cone. I was very excited to get going again until I hit about 2000 rpms, and the same "slip" was happening again. I guess the prop was a symptom, not the problem, any help please. Boat technicians familiar with this unit are hard to find!