Z97 Extreme4 Black Screen issue

Hello,

I have an issue with changing the OC settings in the UEFI. When I run everything @ default or @ default with just the RAM with XMP profile and other non-oc related settings tweaked, everything works fine for years now. System is totally stable, Prime95, Ram, etc. tested and works.

But whenever I try to change anything on the OC settings page in UEFI, .e.g. just set my CPU to manual values, which are not even overclocking (BLCK to 100, or multiplier to 40) I get one of the following issues after saving settings/reboot:

- System boots into windows, but I my screen is black. No changes to recover (e.g. plugging other cable)

- Fans running but DrDebug hangs either at B2, A9, F4 or even nothing. Screen is black.

- Will boot only the first time into windows but then when restarting, one of the above issues appear.

Only reverting to defaults/ cmos clear/recover after x failures solves the issue. Same issue with only 1 RAM or iGPU. I can't event see the Post messages/logo nor getting into the UEFI.

MB: Asrock Z97 Extreme4 (tried either with Bios P1.30 or P2.50)

CPU: Intel i7 4790k

GPU: AMD R290 4GB on PCIE2 (first x16), connected either via DP or Hdmi), watercooled, so I can't check if GPU boots fine (e.g. Fan running)

RAM: 2x Crucial 8GB F3-2400C10-16GTX

SSD: Crucial M500 500GB on Sata3_0

PSU: Seasonic X460 Fanless

OS: Win7 x64, up2date

Monitor: Dell U2711 (with DDC/CI on or deactivated)

With further googling I found someone with the identical problem: http://www.tomshardware.co.uk/answers/id-3659424/cpu-overclocking-problem.html

Sadly, no solution was found. 

I unplugged the external GPU and use the mainboard DP port while overclocking the CPU a little bit. As far as i tested and played arround a bit, it boots without any issues (arround ~15 restarts so far).

I searched the internet for issues regarding the stock AMD R9 290 but couldn't find anything specifically pointing to my problem. There are a lot of issues with black screens already booted in the OS randomly appearing or with 290(X) cards with uefi button switch.

Any ideas which UEFI mainboard settings i could choose to force compatability with my graphics card. I'm relatively sure that both components aren't defective. I thought I already set them all to Legacy mode. Maybe it is the setup timeout which I set to 1. I have to test that aswell.