Display flashes to black

Hi - I have a late 2011 MacBook Pro 17" connected to a late 2008 Apple LED 24" Cinema Display via the dispaly port. The issue I have is that my Cinema Display will intermittently flash to black all the time, very annoying! When I connect may MacBook to my display at work to an old alluminium 21" Apple display theres no issue?

Any ideas whats causing this and what I would need to do to fix it please ....