I have twin (2008) 8.1Ltr Volvo Penta Engines that were recently serviced with new plugs, fuel water separators etc. On my second full tank of gas I experienced low power with WOT on port side, and the starboard side would rev up to 3KRPMs and then idle back down to idle rpms I changed the fuel filters, and was able to squeeze out a bit more rpms on port but not starboard. What could cause the rpms to go up and then down? What could be the problem a bad and weakening fuel pump?