I recently had this problem with the throttle cable on my gp.
Absolutely right in adjusting the cables properly etc.
However my throttle cable was one of the cheap universal ones and there was two much play where the cable sat in the hole on the headset. This was causing the inner to rub and break.
I bought a quality nylon lined one and it fitted better but not perfect!
The only other option is to take the part out of the bottom headset where the cables go into and the hole can be made smaller or larger by tapping with a small hammer or levering with a screwdriver. Dont go to mad or it will break!
You can also bye new ones of these as i think its a fairly common problem.
