Screen flickering: diagnosing the problem

My iMac has had problems with screen flickering for a long while and I am attempting to fix the issue. At the moment, I am trying to understand what the problem is.

When the computer is switched on, the screen operates normally until it starts to heat up. The screen will show some horizontal lines that appear randomly across the screen, which flickers. As the computer heats up further, black blocks appear and the screen starts blinking.

If I play a computer game, the flickering issue is almost non existent. If I connect to an external monitor, there is no flickering at all.

It is my thinking that the GPU and video card are working just fine but there are problems with the link between the video card at the internal monitor, which is affected by heat. But I can't figure out which part to replace.