Polaris XCF 440, once it warms up, from a stand still the tach hits 5000rpm then bogs. clutches are new. any?

If I lift the back of the sled it will rev and run fine till I slow down. I have checked carbs, their fine. if the sled is cold it will rev up and take off fine, but after a little while it starts doing it again. poss. reeds?