Any tips for dealing with backfiring?

[Marjenks]

My 5.7 Mercruiser MPI is backfiring causing an intermittent loss of power when I'm cruising.

It did happen a while back but went away over the summer leading me to think that maybe some moisture was in the fuel now or something minor that just needed the boat to be run, but now I'm thinking it's something servicable that I need to adjust or replace.

Has anyone successfully dealt with this problem and want to offer any tips?

Thanks

[Ewok]

My first reaction is to try a can of seafoam but what are the symptoms, does it happen on engine start or when throttling up from idle or at speed accelerating or coming off plane?

[Marjenks]

It happens at speed. Doesn't seem to happen when idling and not when starting either.

[boatwakes]

Several possibilities: sporadic timing (malfunctioning knock sensor), bad MAP sensor, intake valve not closing, injector stuck open... when it backfires does it only do it once or do you hear a machine gun? Last tune up, ignition module changed ever, hours, fuel pump ever changed?