Crusader 502XL 1996 Surge

[Diligence1]

Hi-I have a Carver 440 1996 with Crusader 502's, the port motor will idle and maneuver at idle but will surge under load and not maintain rpm's or go above 26-2700. I had the service department come to change fuel filters and clean/replace separator (Racor 47047) and they said it was a bad plug wire and replaced it? I took her out last night and she ran great up to max rpm and steady cruise speeds smooth for about 5 nautical miles and then started to sure drop RPMs to idle and back up surge, it would still idle but would surge under a load. I know the gas is old and the Racor element I pulled was terrible (I guess they did not do what I asked them to) I have recently purchased this boat from a widow and it was very well kept but in the last two and a half years she has stayed at the dock clean but no maintenance by the first mate who used her as a floating condo. Any help would be appreciated. I am searching for replacement racor separator elements now DFW area. Thank you in advance.