Open the monitor OSD (On Screen Display) -Display. Is MST set to off? Does the issue occur if you attempt to wake the monitor immediately after it enters sleep? Or are you waiting X amount of time? When it occurs, what happens if you simply disconnect the mDP to DP cable from the monitor and reconnect it?

What revision is your monitor?  I just got this same monitor a month ago, and was leery about this sleep/wake issue because it is pretty widely reported.  However, the newer version which I (luckily) got (revision A01 - Mar 2015) supposedly was better or fixed with an firmware update.  I took the chance and bought this monitor because it's a great price and had all the features I was looking for.

I have a MSI Gaming GTX 980 4GB graphics card and a i7 4790k proc.  I decided to use a regular Display Port to Display Port cable since I already had one rather than the included mini DP to DP.  I don't have the sleep/wake issue.  The monitor works great.

In fact I have purposely tried to recreate this issue you mention and it doesn't happen.  No problems when walking from sleep, or turning off monitor.  Stays at 3840x2160 at 60Hz.

My MST is set to off, and I can wake the monitor immediately after it goes to sleep, or wait and hour.  It works fine.

If you do have the new revision A01, then something else is going on here.  Could it maybe be AMD graphics cards only?  Has anyone with an Nvidia card had this issue?  

I know it must be frustrating for you, as this is a great monitor.  I really love it!  Hope Chris M can help.

**just a thought...do you have any device plugged into any of the monitor's USB ports?  I do not.  Could that be an issue?

I get the same message about link failure. The monitor also flickers.

Reason line = Monitor model, short issue description
Body =
* What is the 20 digit alpha-numeric monitor serial number? CZ-0F525M-73608-021-341L A01
* What specific computer model?
* What operating system?
* What video card?
* Video card driver version?
* What video out ports are available on the video card?
* What video out port are you using from the video card to what video in port on the monitor?
* What cable are you using to attach the video card to the monitor?
* What windows power plan? Example = High performance, PCI-E: Link State Off
* Press the monitor Menu button to open the monitor OSD (On Screen Display). The bottom of that screen should show a resolution. What is listed?
* Was the monitor purchased from Dell or some reseller?

Hello Chris, hope you are still around.  There doesn't appear to be a resolution posted for this issue and guess what, I've just bought a P2715Q and I'm getting the same problems.  It's very frustrating; currently running at a lower resolution after waking from sleep and getting the error message 'The system has detected a link failure and cannot set the requested resolution..........'.  You know the score.  Anyway, hope you can help.  Answers to your questions below:

Reason line = Monitor model, short issue description

Body = See above

* What is the 20 digit alpha-numeric monitor serial number?

CN-032R1F-74445-52C-AV8L  REV A01

* What specific computer model?

ASUS platform on ASUS M4A785TD-V EVO motherboad with AMD Phenom II x2 550 Processor

* What operating system?

Windows 7, 64 bit, latest update

* What video card?

AMD Radeon R7 360 series

* Video card driver version?

15.150.1004.0

Display driver version 2.0.0.0

* What video out ports are available on the video card?

DVI, HDMI, DP

* What video out port are you using from the video card to what video in port on the monitor?

DP to DP

* What cable are you using to attach the video card to the monitor?

Upgraded cable, not one from box.

* What windows power plan? Example = High performance, PCI-E: Link State Off

Balanced

* Press the monitor Menu button to open the monitor OSD (On Screen Display). The bottom of that screen should show a resolution. What is listed?

2560x1440 6-Hz

* Was the monitor purchased from Dell or some reseller?

Reseller.

Read through this first. If troubleshooting cannot resolve the issue and the P2715Q revision is A00, A01, or A02, then Dell Technical Support for your region should setup a used exchange for an A03 revision of the display. A used exchange is the only way to get the A03. For your region, the part number they should order is V48W2 or V7WP9.

Hi Chris, I've had a reply from another member who's had the same issue, also in the EU.  He managed to get a new replacement monitor (eventually) because, like me, the monitor is less than a week old.  Do you have any contact numbers in Dell I can call to start the process for replacement?

Hi, forgot to mention that I tried the troubleshooting topic (no joy, there wasn't anything in there I hadn't tried already - all drivers up to date, graphics card good for 4K at 60 Hz, etc......), so looks like I've got a faulty monitor hence my previous post.  I'm still looking through the support pages; it's not exactly easy finding a number in the EU.  Is there one?

Thanks.  I've been in touch with Customer Services / Technical and explained the situation.  I went through troubleshooting again, to no avail.  The chap I spoke to said they could replace the monitor but they would need the Dell order number from the reseller i.e. the order they placed when purchasing the monitor!  That didn't seem right.  Anyway, I spoke to the reseller and they said the monitors were bought directly from Dell (no date given) but they couldn't provide the order number for that batch (it was a technical person I was talking to so they wouldn't have that information).  They told me my invoice should be good enough as proof of purchase.

To be honest the chap I was talking to didn't really understand the problem.  I explained it was a known problem so he probably looked it up.  He didn't appear understand the Rev A00 to A03 fix so if I got a replacement it could be the same Rev.

He said he file a support request ticket and email a summary of issue, but I've yet to see either.