Hello there, I've just bought a nice road bike but the previous owner had Shimano 600 8 speed derailleur installed working with 6-speed freewheel. As a result the shifting is not perfect - 2 times the chain gets in between the cogs on in the freewheel - is there any way to fix it other than changing the rear wheel that is? Shiftmate? Fancy cable routing? Any ideas please?