took my 1648 out today with its new powerplant.1998 25hp evinrude.while we were testing things out i noticed after about 3/4 throttle the motor revs up but acually slows the boat down...then catches itself and keeps doing it untill i back off the throttle a little?? if i keep it about 3/4 or under she scoots along great! should i be concerned? thanks fellas