I'm trying to help my friend with his sled, so I'll try and describe his problem the best I can.
He has a 98 MXZ 500 and was riding one day when it had a definite bog under heavy throttle. Then once up to top end it ran fine, but all at once it just died "as if it was blown up or something". Waits a few minutes later, and the thing fires right up and seems to run fine. Now since then the sled will only go 80 while still turning 8000 (when it used to go 90). The compression is fine and the belt is fine (according to him).
My thought was that it was two seperate problems. The first being he just overheated it or maybe vapor locked it (idk) and the second being definitely something in the clutching.
Sounds like he could have a spark issue maybe? For the only going 80 instead of the previous 90, that seems like clutching to me also if he was still running the same 8,000 rpm's. My cousin had weak spark on his 98 formula 500 (basically the same sled) and it would bog out and almost die on occasion but other times it would run fine. I just learned about this problem the other day...go figure. He said they replaced the ignition coils and it solved the issue. It could also be as simple as bad plugs. My RXL would seriously only run about 25 mph once when I had bad plugs.