Ski-doo's have a rev limiter... i thought it was set closer to 9k+ rpms.. Anyhow if you hit the rev limiter you're well out of the power range (the way it works is the computer will delay or retard the spark just like on a 4-stroke dirt bike)... Sounds like a carb issue to me...dirty or restricted main jet, jeted to lean, float too low, DPM hose missing, loose or cracked....but it could also be bad reeds and check your carb botts (especially since you've got an 04)...The 04's also had flaky rings (which probably isn't your issue since you say it runs fine if you just back off from wfo)...