I have a 2005 Moomba with the Indamar 5.7 engine and we are having a similar issue.

We changed:
- New Fuel pump
- New Plugs
- New Rotor
- New Belt

Engine went perfect for about 1 hr after all this but then at the end of the day started to lose RPMS again.

Any ideas? I'm think TPS or water/debris in Fuel tank.