Black Scorpion Question

I have a 2005 Sanger V215 with 330hp Black Scorpion. It idles fine and sounds great getting up to wakeboarding speed. Once moving at 17-22mph, it makes popping noises out the exhaust. The RPM's reduce slightly when the pop is heard. Perfect pass usually kicks in adding throttle and the popping goes away. It seems to pop when reducing throttle. At the end of last season, I took it in to a mercruiser dealer. He said it needed a tune up. Changed cap, rotor and plugs. The popping wasn't as loud, but did not go away. They said the computer did not read any problems. I had another mechanic come to the lake with a computer to hear it under load. His reading on the computer was that the MAP sensor was reading 9 for barometric pressure. We replaced the MAP sensor and the Throttle position sensor. MAP sensor was now reading 13 for pressure. I'm not sure what this is supposed to read. I called the mercruiser dealer and he said this reading can vary by boat. When we took it out for a drive under load, the sound continued. He told me that the sound was from water in the gas and that it would work it's way through. I pulled the fuel water seperator and there was no water in it. I replaced it as well. Next step will probably be to change the inline fuel filter. Is it possible that I have an intake manifold gasket leak? I would think that extra air running through the intake manifold would lean it out and cause it to idle at a higher RPM ro there would be other symptoms. It's only got 150 hours on it, so I would hope that's not the issue. Anyone else had a similar issue? Any ideas? Summer is here and I'd sure like to get it running better. It's really frustrating when eveyone says the computer readings look OK but it's not running well.

Thanks.