I had the same on my merc 6.2 twice. First time was a bad plug wire at only 2 yrs old; second was the cap (for the same plug as before) the next season. Thus, I would change the plugs and wires. While you are at it, run a compression check, Easy enough to do, and if everything is OK, it at least gives you a baseline.
Might not hurt to check your fuel filter/fuel pressure. Fuel starvation could be an issue at high load/RPM.
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Walt B
"Debt-Charged"Black '07 H210SS6.2L Bravo One turning a 21P HIFive
Wakeboard Tower with Perfect Pass Stargazer version
Missouri River; Kansas City, Mo.