Have you got a manual? It's all explained in there how to adjust at the lower end of the cable. I also wrote how to do it on here a couple of weeks back, not sure where though?
If it doesn't sort it you may have a frayed/knackered cable.
If that's the case then i would suggest only buying an original Yamaha cable, and when you have unhooked both ends, tape the new one to the old one before removing it so that you can pull it through and it will be routed correctly in the same way. If you start guessing where it goes it could cause you more problems.
After adjusting the lower end first, then the top end, hopefully it will sort it out.
This is definitely worth a read while you're at it.
http://foc-u.co.uk/index.php/topic,10124.25.html