I have a 1969 starcraft with 120hp mercruiser I/O. While I was running up the lake the boat felt like it would come out of gear, and would rev up without going anywhere. I could go slow up the lake at about 2000-2500rpm with no problem. After fishing and running on and off slow all day, the boat operated just fine on the way back to the ramp. Any tips on what this may be?