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Check all grounds on the motor. I had an older merc 2 stroke a few years back that did almost the same thing, took me weeks to figure it out. I thought it was a fuel problem also & did about what you did in fuel lines & plugs etc... I talked to a mechanic buddy of mine, first thing he said ...did you check all grounds to the motor! Sure enough I did & there was one that went to the block that I could tighten by hand. Couple turns on the wrench & problem solved! Not saying it is that, but might be worth looking into.