Help with DF-25, service can't solve it

[al_suzi]

This summer my Suzuki DF-25 Twin from 2006 had following failure:

After some miles at regular speed when I lowered gas at almost idle the warning buzzer and light started intermitently. If it tried to raise gas it started to fail and couldn't go faster.
I stopped motor, waited for around 5 minutes and could start fine and operate normally.
Once I lowered gas again the same symptoms.
Water temperature was fine and the motor seemed not to be warm. Oil level was fine and recently changed.

I started a thread in the forum and was suggested to make a computerized diagnostic.

I went to the service, mechanic told me that motor had water in oil ( I'm not sure about this because I never noticed) and replaced some joints and other parts. He told me this motors doesn't have a computer connection for diagnostic.
After this changes the motor had the same failure.
Then he changed oil pump because he noticed abnormal low pressure after some minutes running.
But this has neither solved the failure. The motor is now more than 2 months at service.

I would really appreciate some help because people at service seems to be unable to solve this issue and I fear they only change pieces without a clear diagnostic.

This motor was never run at high speed or abused, it was almost new when I purchased it and has very few hours on it.

[boats.net]

It would either be low oil pressure or overheat. Probably need to run a compression and leak down test to see what condition engine is in, recheck oil pressure to see if new pump corrected low pressure problem. I would replace water pump, check themostat as well if all the above is good.

[al_suzi]

Thanks, new pump didn't resolved the problem, oil pressure is again low after a few minutes.
May be the oil relief valve or oil input to pump dirty ?

[boats.net]

Yes could relief valve or pickup screen dirty or internal engine problem.

[al_suzi]

He has checked valve and pickup screen and seem ok.

Now he wants to rectify crankshaft and change conrods, this engine has no conrod bearings and therefore conrods must be changed.

In the part list they are 2 conrods, red and no paint, which is the difference between them ?

Thanks