Volvo Penta B20 AQ130D acceleration issue

[Have boat can float]

My Volvo Penta B20 AQ130D has presented with a puzzling acceleration issue. The motor starts and idles perfectly however on accelerating in gear it fails to reach full throttle. It seems to surge back and forward 3 to 5 times over a period of 5 minutes then assumes normal throttle condition. The engine then seems to run fine for a period of time (2 to 3 hours) then without notice at mid to full throttle revs drop to below 3000rpm sometimes to the point of stalling. I have thoroughly and carefully rebuilt the twin Solex 44PAI's with new kits, completely the fuel/water separator and filters, and replaced all ignition components. I have now run out of ideas so looking for some assistance.

 

[alldodge]

The initial take off could be accelerator pump
Loosing power over time can be a weak fuel pump, restriction between the tank and pump (collapsed fuel line, clogged antisiphon valve or tank vent line)