what EK waterblock is it? I had exactly the same problem when I first did my loop. I could not put my core clock higher than 1000MHz or it would black screen, but when they were on air they were happily doing 1100MHz on the core and never saw a black screen ever. The strange thing was, in turning off my pump with the core clock higher than 1000MHz, I never got 1 black screen. Obviously temps rose very quickly so I could only run this test at 10 seconds each time, but I must have tested like this 30 times, never saw 1 black screen. With the pump running again, doing the same test, as soon as anything 3d started, I got a blackscreen. I got another pump from OCUK and the problem went away. But there is a but. On crossfire I can run anything up to 1150MHz on the core now with no black screens, but when only using 1 card, I have to knock the core clock down to 1100MHz or I get those black screens again. It's a strange problem it really is, it's something linked with the EK blocks, the GPU's and the pump that's for sure, maybe frequency issue or something messing around with DP. I don't get any black screens over HDMI no matter what clock speed I set, it's just linked to DP I think.
For ref I use the EK Thermosphere waterblocks on both my 390's, and XSPC 5 Photon 170 Reservoir/Pump Combo with Mayhem pastel white on a BenQ 144hz Freesync screen. You can find a very detailed log of this issue here.
http://forums.overclockers.co.uk/showthread.php?t=18694093 The OCUK members really helped me a lot, was pretty stressful.
