Hello all,
Just going off what LuckyMan94 was saying. i just bought a 27" iMac i5 Thunderbolt display and immediately starting having flicker issues when dragging a file from a folder onto the desktop. The flickering mostly came from icons already on my desktop and sometimes i would get weird pixelated boxes for a split second. Very frustrating to have something like this on a brand new computer. So i tried changing the desktop image like LuckyMan94 suggested and noticed that the flickering wasn't as bad with some images but it was still present nonetheless.
Then i had a brainstorm when i noticed that 2 icons on the desktop were the worst offenders. what if i just moved them to a different location on the desktop? VOILA! They stopped flickering. So the logical conclusion hit me like a thunderbolt (pun) -- what if i just shift all icons and move them back? Problem completely solved.
Now i should mention that I set up my new computer with my most recent Time Machine backup from my older system. This just replaced all of the icons i had on my old comp to the new desktop. I have a feeling the system was having some kind of conflict with the new screen orientation or something. Either way i felt compelled to share as I know how frustrating this is and maybe it'll help. It would also prevent wasted time and energy running back and forth to the apple store having them replace your whole system and you come home to restore and have exactly the same problem. Give it a try.