Superjet problems

im having problems with my 2004 superjet. The ski will do maybe 75% power in the water. I can't get to top speed. I changed the fuel lines, checked the carbs they're good, new plugs and spark plug boots and nothing changed. Riding it yesterday and out of no where the ski rode like a bat out of hell. Was running beautify. Throttle was 100% responsive then next thing you know it's back to riding how it was. I take it out of the water and bogs when I hammer it then back fires like crazy when it's at full throttle.
 
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