I have a Trek 800 with an 8 speed Cassette. I have worked on a lot of bikes , but I have never run into this problem. The Bike shifts into all gears from the lowest to the 7th gear. When it gets to the 8th gear the chain does not match the teeth of the gears, caused the chain to ride on top of some of the teeth. I thought the chain was worn, so I replaced it and it actually became worse. I took the cassette off and checked highest gear to make sure it was placed on right, its was and it can only fit in one position. I looked at gears from side to make sure the highest gear wasn't bent, and its straight and not causing chain to hang up. Its almost like that high gear is not the right spacing for a regular chain. I was going to try another gear to see if that fixes it. Just wondered if anyone ever had this problem before?