Looking for any mechanical help from the experts. So I have a 2003 Mercruiser v8 5.0l MPI. The motor is having fuel pressure issues. It runs great at top end. 3600-4000 rpm. After the testing I've done we narrowed it down to being either a fuel pump. Or fuel return regulator. The boat starts great but then begins to lose fuel pressure. (Bleeding back the return regulator???) The voltage holds at 14 so I know we're good there. Is there any way to know which one it is that's bad? I don't want to buy a pump and have it be the regulator. Or visa versa. Just curious if anyone knows any tricks to testing the two. Also it seems to start and hold fine cold but after the motor warms up is when pressure begins to go down which is why I think it could be the fuel return regulator. Also the fuel pressure gauge doesn't hold steady like on a car. It bounces back and forth. Any input or help would be appreciated. You can pm or call or text me. Thanks.