Tachometer sticking

Does anyone know how to fix a sticking tachometer it works fine then after i go over 4k and come back under 4k it sticks at 4k until i tap the tach any idea how to fix ?

Mechanical or electrical, what yr./model bike is it ??
If mechanical, remove cable at both ends & inspect inner square cable to make sure it's not broke some where
also grease the cable if not broke
Pete

Its a 1979 kz 650 when i get some freetime ill try that the. Where the cable goes into the engine block is really loose and covered in gas and dirt this is probally the cause dont you think? What should i clean cable with and then what should i oil it with?