Volvo Penta MD 2030 not holding rpm?

I have a Catamaran sail boat with two identical volvo penta MD 2030. The starboard engine works perfectly. The port engine starts up well but the rpm

I have a Catamaran sail boat with two identical volvo penta MD 2030. The starboard engine works perfectly. The port engine starts up well but the rpm does not increase with the throttle open. Sometimes after 30 seconds the rpm increases say to 1500 rpm. It may work for a while sometimes seconds sometimes minutes sometimes 2 hours. After running for a while with the throttle at the same position the rpm goes down, sometimes 500 rpm, sometimes to idle speed. It will stay there for a minute sometimes two until it goes back to set rpm.
First I was told it was dirty fuel, but the fuel filter was clean. Then I suspected that air enters the system. When bleeding the engine the fuel comes immediately.
So far I have blown out the copper fuel line leading to the tank about 15 ft away. I have polished the fuel, fuel tank removed and cleaned. Replaced the rubber fuel line leading to the engine and returning to the tank (part that is not copper), replaced the fuel filter with a Racor 120AS. I have installed an inline fuel pump.
Please advise me how to fix the problem.

or:I have a Catamaran sail boat with two identical volvo penta MD 2030. The starboard engine works perfectly. The port engine starts up well but the rpm does not increase with the throttle open. Sometimes after 30 seconds the rpm increases say to 1500 rpm. It may work for a while sometimes seconds sometimes minutes sometimes 2 hours. After running for a while with the throttle at the same position the rpm goes down, sometimes 500 rpm, sometimes to idle speed. It will stay there for a minute sometimes two until it goes back to set rpm.First I was told it was dirty fuel, but the fuel filter was clean. Then I suspected that air enters the system. When bleeding the engine the fuel comes immediately.So far I have blown out the copper fuel line leading to the tank about 15 ft away. I have polished the fuel, fuel tank removed and cleaned. Replaced the rubber fuel line leading to the engine and returning to the tank (part that is not copper), replaced the fuel filter with a Racor 120AS. I have installed an inline fuel pump. Please advise me how to fix the problem.

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