How can I fix this bad contact(with video ) in my 27” apple display?

Sorry to say you need a brain surgeon to fix this!

Basically, the ribbon cables which connect to the glass LCD panel to the T-CON board have lost there bonding!

Think how a loose light bulb rattles in a socket as it makes and breaks contact!

You'll need to find someone with the skills to re-bond the cables to the panel, that is if the pads on the panel are still able to be connected to. Often times the cable pulls the metal oxide printed contact off the glass so there is nothing to connect do. The only reason you are getting a signal is the edges of what's left is making contact.

Then it comes to finding someone and the costs of the repair if its repairable which is not that good ;-{

Frankly, while expensive it's often cheaper and quicker and thats just replacing the full display. The cheapest one I can find is this one on eBay Genuine Apple iMac 27" LCD Display Screen LM270WQ1 (SD) (B3) Just becareful! Use a prepaid card ands one that offers buyers insurance.