Guys, it's not actually the overclocking that's causing the flickering, it's the stupid idling speed that causes all the flickering issues-the card actually will flicker less if you overclock using a method that forces the card to always run high clocks.
This is right. There's some confusion in this thread, so for clarification:
1) A 5XXX card can drive one monitor at 157/300, but requires substantially more juice (usually 400/1200) to drive two or more. Trying to use two monitors at 157/300 causes weird flickering and tearing on the desktop.
2) The drivers are aware of this and will automatically only clock down to 400/1200 when you have two or more monitors plugged in.
3)
There is a bug in the drivers that causes the card to revert to idle clocks of 157/300 whenever you overclock (or underclock for that matter), no matter how tiny the change you make is.
4) This is the cause of the flickering. It has nothing to do with the clocks you set. When you tried to overclock to from 850/1200 to 855/1210, your 2D mode card instantly dropped from 400/1200 to 157/300, causing instability on your desktop.
5) If you want to verify this is the cause, do your overclocking with a game running in a window in the background. No flickering.
6) The solution I posted above, which involves setting a new profile for the 2D clocks, stops this bug from occurring, and you can then happily overclock with whatever tool you like.
7) In my experience MSI Afterburner
sometimes gets around this problem and lets you overclock without changing the 2D clocks, but not always. It's a more consistent fix to do it in CCC.
8) No, you didn't get a card that was binned so close to the limits that a 5 MHz bump in core speed caused it to spaz out.
Interesting I have an issue where if I attempt to overclock even 10mhz i get flickering. The card I got from Asus already idles at 400/1200.
You're saying you had this issue and had to increase it to stop it?
I had exactly this issue and thought I'd got a dud. Fixed it and it now clocks happily at about 960/1400, like pretty much every other 5770 out there. Like I said, just leave a game or Furmark or something running while you try overclocking. I bet you it'll work fine. A card that had problems on the desktop at a 5MHz overclock wouldn't last half a second in a 3D game at its stock clocks, but I'm sure you're not having any stability issues in games. Believe me, the 2D clocks are the problem.