There are only two gears... Forward & Reverse.
Your problem... If it feels like you're hitting a submerged object, the lower unit is jumping out of and back into forward gear.
If however, it does not feel like you're hitting anything but rather the engine simply revs up to a higher rpm and the boat slows down... that is usually caused by a slipping hub within the propeller. The propeller is actually a three (3) piece unit.... the propeller itself, a vaulcanized rubber section within the propeller, then a bronze hub within the vaulcanized rubber. The rubber fails and allows the bronze hub, which is splined to the propshaft, to slip within the propeller and is referred to as a.... slipping hub.
If this is the case or your suspicion, remove the prop nut and draw a line with a magic marker on the bronze hub and the propeller. Replace the prop nut, run the engine to encounter the problem once more, now remove the prop nut again and observe the markings. If the mark on the hub is out of alignment with the mark on the prop, then obviously the hub is slipping. The cure would be to either have the prop re-hubbed or install another propeller.