my dad has a 30 volvo penta 8.1 ltr and it some times does that same thing like you will be goin and it just jerks and the rps's drop (is that what is happening to yours) but if that is the problem i would check your air filter change it if you need to and maybe put some octane booster in it and possibly think about getting a new gas cap if your boat is in board i would also check your fuel pump and filter hope this helps