I know that there are many discussions about the DP/mDP daisy chain issues with this, and other, Dell monitors, but my issue seems to be a bit different.
I purchased three U2415 from Dell and when using the DP daisy chain feature had issues with screens going black for a second or two, at random intervals. Tried all the different fixes that the customer service rep suggested over a couple hours phone call, but they it didn't seem to change anything. At the same time I noted that one of the monitors had a noticeably different colour than the other two and Dell ended up sending three new monitors. I only replaced one of the monitors as the first one I tried didn't have the same color issue.
Anyway, I currently don't have any issue with the monitors not "waking" when I start the computer, but I do continue to have the issues with monitors going black for a couple seconds. This may not happen for 15-20 minutes, or it might be a couple times a minute. The part that is different from what I've read is that it only happens to the second and third monitors in the chain; it never happens to the first monitor. Monitors 2 and 3 may go black at the same time (most common), or only one might go black.
Thinking that the issue may have been with my older XFX Radeon HD6870 I replaced it with an Asus Geforce GTX960 a couple weeks ago, but obviously the issue persists. My system is running Windows 7 Professional 64, with 24GB RAM, and an Asus Sabertooth X58 i7.
When I started having these issues I contacted Dell, as mentioned above, and the rep said this flickering was common to all DisplayPort installations. I next contacted DisplayPort.org directly and explained the situation got this reply: "I have seen this on some newer configurations but only a few. It is not obvious yet which combinations of source and monitor have this problem. But it is not endemic to DisplayPort and is not "the way it is". The problem is related to a few product implementations."
I don't have a great deal of experience with computer hardware issues, but with a rock solid display on the first monitor, and only experiencing the problem on downstream monitors it sure seems like the issue is with how the Dell monitors are handling DisplayPort. With the number of threads complaining about DisplayPort issues on various Dell monitors, it would sure be nice if this was something that could be resolved. In the meantime I've just resigned myself to doing most of my work on Monitor 1 and just taking a deep breath every time the other two black out.