Boat motor dies after throttling down?

my father has a mercury 40 hp 4 stroke its pretty much brand new. its on a pontoon boat...but anyways heres the problem, every time he uses it at higher rpms it workd great but its when he throttles down to low rpms or idle it shuts off almost every time. it starts right back up though without hesitation. any good info would be very helpful. also i know it has three carbs if that helps any