Directly connected through supplied Dell MiniDisplayPort to DisplayPort cable. Problem occurs only at the display’s native resolution of 2,560 x 1,440 px. Restarting the Mac most of the time resolves the problem. One then can work for hours, but if the Mac goes to sleep, chance are high that the Dell U2515H begins to flicker again on wake up.
If reduced to lower (interpolated) resolution (e.g. 1,080p) problem is gone. No problems at full native res on new MacBook Pro Retina (Late 2013).
This problem occurs with both of my U2515H displays. Swapped the mDP-DP cables, no improvement either.
Ideas anyone?