Why is my display flickering after this repair?

--2/14/2013--

After completing this DIY, I am seeing occasional (2-3 times per minute) flickering / dimming of the iMac display. It happens for ~1 second and only dims the image; the screen does not turn off completely nor is there any distortion of the image. I can post a video if that is helpful.

I was pretty careful to ensure the display cable was properly secured, and the screws holding the display in are uniformly snug. (I saw some reports for earlier iMac generations that suggested display issues could result from improperly secured screws.)

I should mention that I undertook this upgrade shortly after buying the iMac used, so it is possible that the issue was already there and I didn't notice it before beginning surgery. But I had used it for a few hours and never saw it, so I'm skeptical that the problem existed before the upgrade.

Any ideas?

--2/16/2013--

Tried removing the display again, re-seated all display connectors, reinstalled display. The issue persists.