As risk of stating the obvious, have you tried to replace your old cables?
Assuming they are stainless steel cables, the amount of stretching should
be a fraction of an inch, which I'm guess you've long ago exceeded.
The cables are less than 2years old and I have tightened each side approx. 1/2 " in the last 2800nm (1 year). However, the amount of adjustment needed gets less everytime. Initial stretch of cables? Cables made in $#@& ? How often should they be replaced?