Hello,
The internal GFX shares the same voltage supply as the memory controller so when you enable the XMP you are also automatically over-volting the GPU which needs near default voltages to function correctly. (If you didn't know, the XMP also changes voltages not just timing / frequency)
VccSA = 1.05v
VccIO = 0.95v
These are default values which the GPU requires but for the memory controller running over 2133Mhz most XMP set these voltages over 1.25v.
You will have to choose some values and see if the screen flickering stops but if you go to low your 3400Mhz OC might become unstable. Please note your board will have an offset, whatever voltage you choose it will not run at that, usually slightly over. Eg, you choose 1.10v in the BIOS, applied voltage might be 1.16v.
So where to start?
VccSA = 1.10v <System Agent Voltage
VccIO = 1.10v
Might work for both GPU and 3400Mhz.
I tried to slow down the speed of memories - 3200/3333/3000, but it seems that causes instability. I have the same parts and the same problem, activate the xmp with DDR4 3400 GSkill the screen starts flashing in the windows by hdmi. Sometimes windows files are corrupted, some dll, I have to format.
I tried to change the values of VCCIO and VCCSA to 1.05 to 1.1, with the memories of 3200 (32x100) worked well for about 6 hours, suddenly, no rar could be unzipped, videos in mkv locking, problem with music closing by themselves, the rest normal functioning. I tried with 1:15 on both with the memories in 3333mhz and windows files have been corrupted, sometimes the windows comes with a temporary profile.
Well, I'll try 3400 (33.33 x 102 - which is the default) and VCCIO and vvcsa 1.15, it does not work, I will try to exchange the memories. I noticed that in the automatic mode is 33.33x102 VCCIO and VCCSA 1.2 with bios f8l and the VCCIO f7 1.2 and 1.3 VCCSA but this way the screen flashes with integrated Video i7 6700K.
I can not do the Vcore automatically decreases, only to let the CPU Vcore in automatic if manually put any value it fixed and not down more, even the clock decreasing.