Originally Posted by jAkUp
It is because you are unlinked, you can only push the core clock so high with an unlinked shader.
I have a heavily overclocked GTX 260 and thats not true, unlinked is actually better as linked will have you hitting a wall much sooner with the shader before you ever find your max core.
The best way to find your max OC with the GTX 260 is to unlink the shader/core and then find the max shader clock first, after finding the max shader go for your max core.
Example my shader topped out at 1536mhz and this allowed me to find my max core of 760mhz. If I keep a linked ratio it puts my shader at 1636 which is way to high.