Connecting a Dell U2412M to a 2013 Retina MacBook Pro using DisplayPort to Mini DisplayPort
I had the exact same problem with a Dell monitor.  After months of support calls and monitor replacements, it ended up being the cable.

The problem was using a Mini-DP to DP cable.  For some reason, the DP chips in the cable is not sending a signal that Dell monitors are expecting or the length of the cable causes the signal to degrade.

The fix? Buy a Mini-DP to DP adapter then use a 1.2 DP to DP cable to connect to the monitor.  I know it sounds lame and the MiniDP to DP cable should work but after I switched out the cable for this setup, I didn't experience the issue again.

Hope this helps.