I'm posting this to help anyone else with the same issue as me. I'll repeat the details of my problem here.My issues with this monitor
390X with XL2730Z running 1440p, 144Hz via Display port. I encountered frequent black screens while on the desktop, or in games. The black screen would last for 1-2 seconds, then return to where it left off. The PC wouldn't crash, audio would still play while the screen was black etc.
I also got occasional flickering while on the desktop - a white line near the top of the screen that lasted 0.1 seconds if that.
These issues only occurred while running via DP. I obviously had to run via DP to run 1440p 144Hz and to take advantage of Freesync, so using HDMI or DVI was not an option.
These problems were quite annoying - as I could be playing a competitive game and at a crucial moment I'd get a 2 second black screen - as you could imagine this is rather rage inducing, so this had to be fixed.What I did to try and fix it
I ordered a 2M Accell DP cable to try and fix it - this did reduce the frequency of the black screens and flickering, but they still occurred. Note that the DP cable included with the monitor (Amphenol 2M) is indeed DP certified, though the Accell cables were highly recommended, thus I thought it was worth a go.
I tried running my 390X at full 3D clock speeds. This also reduced the frequency of black screens/flickering, though they still occurred.
I tried overvolting and undervolting my 390X. I also tried underclocking it. This made absolutely no difference.
I tried the 390X in another motherboard with a different CPU. Same issue occuered.
I tried the monitor on another GPU, a 7950 via mini DP. The same issue occurred on the desktop. I couldn't really test in games as the 7950 couldn't run freesync and produced too low FPS to bother testing.The final solution
I decided to try a 1M display port cable. I saw that AMDmatt was recommending this cable to several people in various threads, though there is only one place in the UK stocking this (that I could find) and they were out of stock until 27th of December.
I also saw that AMD are recommending a less than 2M DP cable for their next generation GPU's that will use DP 1.3:
I understand they are talking about next gen GPU's with DP 1.3, though this made me curious as to why they were specifically recommending a <2M cable.
The 1M Accell cable arrived on the 28th of December, and I'm happy to report that I've not had a single black screen or flickering issue since. I've used the PC for a good fews hours every day since, so I'm 100% confident that this is a permanent fix.
Hopefully this might help some other poor soul who's having these frustrating problems out there.
Here's the exact model number of the cable:
"Accell B142C-003B" DisplayPort to DP Latching Cable
Now that my issues with this monitor are resolved, I'm extremely happy with my purchase. The monitor is simply excellent, 1440P 144Hz Freesync is stunning