if all that checks out as well as you say, it's time to check compression. My wifes 96 XLT fly's up to 85 and gradually goes to 100 (on the speedo). It is bone stock. I'm not a big believer in buying used sleds. I live in northern Mich. I see what conditions those people from downstate ride their sleds in. It's like "I PAYED $6000 FOR THIS SLED AND I'M GOING TO RIDE IT" mentality. Sorry you are having problems with your sled. I still believe the belt needs to be adjusted, just my 2 cents worth.
Runner
Runner