Engine Hesitation in 1994 PCM HO

OK Guru's.This is the first post for a novice owner of a 1994 Sport Nautique(164 hrs) with a 351/5.8 HO. It is electronic/ Fuel injected into a Holley carb.. The problem is it will do anything but maintain a constant speed. Especially in the 20-30 mph range.It will be running 22 and start to slow. RPM's go down, oil pressure ect.. Give it more throttle and it sort of bogs down, then accelerate like crazy and you reduce throttle and do it all over again. Will run 35-40 all day with no hesitation. It is strong out of the hole... I have changed plugs, elect. diode greased everything I can find, changed fuel filter,oil ect. Im afraid the coil pack could be bad and unfortunately unavailable.How to test? The origional owner said its been like this since 2010 after it "Ran Hot". He had abt 1600$ in new left exhaust manifold and labor done and the motor was "sound" at the Nautique dealer. I find myself sitting in the boat a 2 AM lookin for something else to do. I dont mind spending the money to make it right, just dont have a clue as what to do next....