Crusader 5.7 MPI RPM Loss Mystery Please help me find this issue... Lot's of steps taken already

Boat:
Silverton 330 Sport Bridge
2002 Crusader 5.7L MPIs
Raw water cooled.
Both engines running off the same tank.
Motor in question just had heads replaced and timing done with new plugs.

Filtration from Tank to Motor.
1 Hocky puck filter
1 paper filter on high pressure fuel pump in the FCC.

Here are the symptoms:
Boat get's up on plane pretty quick (3600-4000 RPM) and runs the same exact distance (approx 1.5 miles or about 6-7 minutes) before port motor drops RPMs. When this happens it sounds like there is a huge miss in the motor sounds like it's about to shut down. If you bring the throttle down to 2400rpm it will run forever. Once at 2400 RPM for a few seconds, it will go back up to 3600 RPM for a few seconds. The longer you keep it at 2400 RPM the longer you can do 3600 RPM. No alarms go off when the RPM's drop. During all this the starboard motor is a champ, always ready to party so fuel in the tank isn't an issue. Tank vent isn't an issue. When I swap parts, the starboard side is always fine. Port always has the issue.

This is what I've done so far:
Swapped anti-siphon pickups from the tank - no fix.
Swapped pressure regulators on motors - no fix.
Emptied water from Fuel Control Cell and changed filter - no fix
Cleaned Hocky Puck filter (mesh screen) - no fix
Swapped low and high pressure fuel pumps - no fix
inspected all lines between puck filter all the way to the rail - no issue, no fix
disconnected oil pressure sender switch - no fix.

I know... put it on the computer or pressure guage on the fuel lines. But short of that, anyone else face this problem? Anything else you would try?
Is a knock sensor triggering timing adjustment? If so would it run fine for a while?
Map Sensor? How do I test this?
Any help would be appreciated. Let the brilliance begin!

Thank you in advance for your help.

The top end of the engine was off so most of the sensors were disconnected...getting a scanner will let you know if they are connected and functioning correctly...without it, you'll have to verify each one which is very labor intensive.

I'd make sure the secondary side of the ignition is issue free, too.

You caould have an issue in the plumbing between the tank and the pump...if you are careful, you can substitute an outboard tank to suppy your port engine...

You state that the timing was just done, but on that engine the computer does the timing off of the crankshaft sensor and it cannot be changed. The traditional "loosen the distributor and rotate" adjusts the "cam retard" not the timing. Since the distributor was off when the heads were done, your cam retard may be incorrect. What you are doing when you adjust this parameter is making sure the rotor is pointing exactly at the correct conductor for the appropriate spark plug. If off, the spark may be jumping too big of a gap in the distributor cap to sustain high rpm operation. Doesn't exactly fit your symptoms (or maybe it does since I have never had the problem), but the point is cam retard must be reset whenever the distributor is pulled, and unfortunately it can only be reset with a scanner.

Having said that, I have had some luck setting the distributor position by carefully looking at the other "known good one" and turning it until it exactly matches its position. Then when I scan, it is always remarkably close to the right value (within two degrees). It is worth a try.

Beyond that, Google "Pirate 4x4" and read the article on MEFI engine controls. Then follow the link to the manual and read that and all your questions will be answered. It is a 300+ page download so set your printer to double side! And as for your MAP sensor, the manual will show you where it is at (right on top of the intake manifold). That is an easy swap with the other engine, although your symptoms sound even less like that problem than the cam retard suggestion. And check to be sure that all electrical plugs are plugged and tight. It is easy to miss one during a head job. You have done most of the fuel supply stuff so if you can't find it soon, you really do need a scan.