Several things it could be....
If it's a smooth "has bite, now it doesn't" kind of thing, it could be ventilation (like jbuote described above) or it could be the prop hub slipping. Your propeller has an inner hub and an outer hub, joined by a rubber sleeve. That sleeve is designed to slip if you hit something. Over time, the rubber can deteriorate to the point where just putting a load on the prop will allow it to slip. A mark across the propeller and prop nut with a Sharpie, followed by a running test to see if the marks are still aligned can tell you if it's the hub.
If it's a violent "ka-chunk" kind of coming out of gear, I'd lean towards a clutch dog going bad. The dog is the part that engages the gears to the propshaft. As it wears, the lugs get rounded over, allowing them to slip past the mating lugs on the gears. Each time it slips out, it then slips back in with a transom-shaking thunk.
