2001RFI fouling plugs

[glen young]

Just got this ski this year and about every 5 hours it will not run past 4,000rpm, change plugs and them runs great and after 5 hours, it's the same thing again. I getting no messages or anything else, it even starts good when it won't go past 4000rpm. I thought at first it was going into limp mode, but after changing plug fix it, I don't think it is. I've tried unpluging the linard when it was only going at 4000rpm and that didn't do anything, just new plugs fixes it. I tried BRP8ES and BRP7ES it doesn't seen to make any difference.

Any ideas about what's happen or is this normal for a 2001RFI?

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Thanks