27" Cinema Display will not turn on with 27" IMac

I have a 27" cinema display mini dvi type connected to a 27" imac (late 2012, Latest model)

The problem I am having is everytime I turn my IMac on I have to completly unplug my cinema display from my IMac.

If I dont the cinema display will not start.

This problem has only happened since I upgraded from an old 27" IMac to a new one

Any help would be greatly appreciated

Regard

David