For those who are still struggling with the sleep-flicker problem have you tried the verified answer by MarkMessenger to install the Chipset Driver? Solved the problem for me since the solution was posted without using the previous verified answer of changing the C-State which adversely impacts battery usage.
Intel(R) Management Engine Components Installer Driver
Foodlady, others report that they had a problem downloading and installing the Chipset. I also had a problem and had to try several times (finally got it to work at night). At least in my case when it downloaded properly it was 104 MB in size otherwise the size was both larger and smaller. The 104 MB file installed OK and then I turned C-States back on. Have had no problem for several weeks. Was the file that you downloaded 104 MB (as I recall this is the size listed on Dell's download page)? I think that this is important. Rather amazing that Dell cannot provide a download that everybody get OK (maybe something screwed up with their server?). I also have HP computers and they download and install driver updates automatically without a hitch. That's about all that I know - Good Luck!
I deactivated automatic driver updates as described in this thread!
I deinstalled 4256 'deleting software of the Driver' until 'Basic Microsoft Video Adapter' appeared - than restarted and installed 4240 which is flickering a lot less but on the other hand has sometimes problems to wake up from sleep and when it does the UI size adaption and video acceleration is gone so I have to reboot
This morning there was more flickering again and I checked the driver and 4256 was installed AGAIN without me doing nothing.
I also installed the chipset driver (made sure it's 104MB). Installation went flawless. It's seems to be better but not perfect.
To test you can change energy settings so closing the lid will NOT put the laptop to sleep. It needs to sleep at least ones before it starts happening - then just close and open the lid several times.
Released today - driver version 10.18.15.4279 (package 15407.4279) – looks good for the moment.
I thought my problem was resolved, driver is always installing 4256 even after disabling/blocking automatic update of drivers in advance system hardware setting. And flickering goes back, but tolerable as it goes away after about 5 seconds, sometimes just after logging in.
Then I installed the Intel driver mentioned by redbird71 above, video driver is now Intel 10.18.15.4279 dated 8/24/2015. Afterwards, flickering get worse and takes longer before it settled down.
I followed everything that was mentioned in this forum to resolve this issue, some will trick you that it has been fixed but after some time, it'll get back just like the Terminator.
For now, I will disable C-state until this has been resolved. This is only a workaround as it affects battery life.
DELL, PLEASE FIX THIS PROBLEM!