Optimax 225 surges up and down at low speed

I have a 2000 Merc Optimax 225 EFI saltwater on my Whaler and late last season it started doing this strange surging at low speed. Flat out, it is just a fast as ever with no hesitations at all. I have to travel through a couple of miles of no-wake and that's when it is really noticeable. It will slow way down then surge ahead like someone is moving the throttle back and forth. Used to be just annoying but after several trip to two different mechanics it's become downright expensive (replacement parts fishing expeditions). They've replaced the plugs and fuel filter but no change. I even replaced the pressure bulb and fuel lines from the tank thinking it an air leak.
One guy recommended replacing the battery (jeezz!)
Any ideas?