My 96 60 hp Bigfoot engine starts, idles, and runs great wide open but seems to sputter a bit at mid range. When I push the throttle up it will remain at a lower speed and then suddenly "kick-in" as I push the throttle up to full. Need help.......

Look at your carbs.Find the mixture screws.If the upper carbs have mixture screws,and the lower ones don't (no spring on mixture screw),then your motor only idles on the upper cylinders.
The lower carbs start to kick in around 1800 to 2000 RPMs .
Generally this is not a bad thing,but on some boat/motor combinations the boat will actually accelerate quite a bit during this transition.
Being 1996 engine I don't think Merc is obligated to do anything,but they may have some ideas.
New carbs are pricey.