2002 Mercury 15hp 4 stroke

This may sound rather simple, but, is the vent screw open in the fuel tank cap? For proper fuel flow you need ventilation in the fuel tank to avoid a vacuum in it. Fuel won't flow well or not at all in a vacuum. It happened to me, it can happen to you. My 9.9 four stroke Mercury kicker acted just like yours while in a Walleye tournament on Saginaw Bay. I about to head back to the dock. Then suddenly I noticed the vent screw was down. In less than a minute I was back to troll speed. Best wishes.