Intermittent flickering on 24" Cinema Display

When I connect my 24" Cinema Display to my iMac (27" mid-2011 running El Capitan) the Cinema Display goes black for a second or two intermittently. When it's connected to my Macbook Air it works fine.

I have tried downloading the ThunderboltDisplayFWUpdate1.2 as has been suggested on this forum but when I try and install it I get a "need to update software to install this" message. When I try and install a prior update (ThunderboldDisplayFirmwareUpdate) I get a message saying it's already installed. I have read that Apple privately offers an update to fix this specific problem but no longer offers it publicly.

Any help would be much appreciated.

Jay