I have several U2515H in house and do have the occasional issue of U2515H going on and off and or losing sync (digital weirdness on the display). What I have done to correct this every time is to remove the power (physically disconnecting the power cord) and then reconnecting the power cord to the U2515H.
From what I can see, this seems to be a U2515H firmware issue. The U2515H internal intelligence just goes wild. But for now, it is a bearable situation.
I just hope that this is not some bad internal component that is aging prematurely, thus making the issue to appear more frequently like some of you are describing, making this issue unbearable to users.
I would like to possibly have some input from a Dell monitor engineer on this subject.
Is this a firmware or is it a hardware issue?
I am using DP (DisplayPort) with the latest drivers from Nvidia.