This sounds to me like an age related problem.I think that shes going into gear
ok , but nothing much happens till you take her up beyond idle .If this is the case I`m thinking its the forward clutch
that is needing adjustment.These are a simple box to adjust .I`m not sure hoe mechanical you are , or what the model of your box is , and Im reluctant to advise you any further until I know of a model number.You may be able to get some info yourself , just by searching the net , start with paragon marine
gear , and see where it takes you .If you make an adjustment , make it in small increments .Post again with any results . good luck .