1999 5.7 mercruiser (Fuel injected) backfiring issue

[dippasan]

I developed a hesitation and backfire problem in my port engine this past weekend. The Starboard engine is running fine The boat has been running great since launched a month ago. I've put about 8 hours on it so far this year with no problems til Saturday.
Both engines have 360 hours
While cruising at 3000 rpm's the port engine suddenly lost power and started backfiring then seemed to smooth out some but still wasn't right. Seems to run fine at idle but stumbles when in neutral while throttling up.

I did the following work myself this spring to both engines. (I used to be an auto mechanic in my former life):
New plugs
new wires
new caps
new rotors
Oil and filter change
both Fuel filters
Filled with fresh gas on Sunday (Problem occurred before the fill up and no change after the fill up)

Firing order is correct. I removed the cap and rotor and it all looks fine.
It seems to idle smooth but stumbles under load. My first thought was fuel issue but the starboard engine runs great.

Thanks for reading

Any ideas would be appreciated