2008 Optimax 135 hp engine fails at iddle speed

[caribbean boater]

Over last two or three months my engine has not been working properly at low rpm's. When I go in or out the marina at low speed, the engine fails and sometimes shut down completely but I start again without problem. When I go out the marina and I start to navigate at high speed my engine works fine. To resolve the problem I have changed the spark plugs, the impeller, the fuel filter and I checked the spark plug wires which looked fine.

But the failing at low rpm's or iddle speed continues ! I always use the DFI Oil and all Mercury parts.

What I should do? It could be a fuel injectors problem? Could I try a fuel injector cleaner?

Thank you for your suggestions!