Any pump of the correct rating should work. The components that might cause your problem start with the flux gate compass and then the course computer. I have the same autopilot and there are a variety of Raymarine course computers that will work. It sounds like the likely cause is the flux gate compass. It needs to be located low and near amidships. It should also not be near any iron containing items such as pumps, or even steel reinforced hoses. If you get the install manual for your system, there are usually ways to calibrate the flux gate and set up (commission) the autopilot. There are also sensitivity settings that determine how much "slop" the course computer allows and how fast it corrects. You may just need a new flux gate compass at $60-90. If you swap the course computer I'd recommend one with a yaw rate gyro equipped versions. I moved my fluxgate on my 330 away from all metal and swapped my Raymarine S1 course computer and Raymarine Gyro Plus 2 (separate yaw rate gyro) with a used Raymarine SPX10 ($200). It vastly improved my autopilot's tracking and heading control. It doesn't have a lot of fancy trolling patterns, etc., but I kept the same control head and am very pleased with the low cost upgrade. Tracks within feet and heading control is very good.
