Volvo TAMD 40B loosing RPM's

I have a 30' Chriscraft with twin TAMD 40B's. The last two trips I have experienced a problem where the Port engine looses RPM's. I will be cruising at 3200RPM and the port engine suddenly drops to 2900 or 3000, then after a second or two it slowly climbs back up to 3200. It does this every few minutes. Aside from the loss of power, the engine seems fine, temp holds steady, no change in exhaust smoke.

I changed both the main and secondary fuel filters, but problem is still there. Only happens on the port engine, not starboard.

Any ideas what can cause this?

Check the fuel lines and pumps. Check the intake side and change air filters. Could be a whole host of problems I don't know where it start. Have you just changed the timing belt? Is it too tight? Oil pressure fine? To be honest the best thing to do is check the air and fuel then call professional it would be almost impossible to tell without listening to the engine.

Just had a boating friend with a simlair problem or though not exactly
the same, with an AQAD41A engine after having it serviced it ran fine for around 7 days of a 14 trip when his starboard engine won't rev over 2200 rpm, he rang the mechanic who serviced the boat for help was told mostly likely a fuel feed issue, he also asked "did you fit the new air filters in left in the boat be side the engine" - "yes" was the reply,
He tried by passing the fuel feed system to the lift pump with a tote tank but problem was still present. I went down and gave him a hand as he had purchased a new lift pump to fit. I suggested we make sure the turbo was rotating freely took the air cleaner off which was complexly clogged, turbo rotated freely - didn't look like filter had been changed for serval services back we new that they certainly weren't the filters he had purchased and left to be fitted when the service was done as they had different coloured end caps