Hi, just go back to 0904 bios, more stable. I just tried to flashback to 0904, because new bios disable the CEP that's make ur system not stable. When I use 1102 bios, I have memory stability at 7800mhz. And cinebech23 error at stock setting.

Hi, is undervolting cpu affect the memory stability?

XMP 1 is to tight timing for me. Have you check what the different between XMP 1 and 2? You can try both of them. XMP 1 7600, 7400 , 7200 or XMP 2 7600, 7400, 7200 until you system no crash.  Use y-cruncher and memtest to test your stability. 

Yes I set XMP 2 and downclock it to 7800Mhz. It is simple way to get stable. I need more time to tuning the timing and voltage to get stable. 

Unstable memory, u need to tuning your memory. I got same problem as you. Don't expect use high speed memory with XMP only. I have g.skill 8200Mhz can't stable even at 8000mhz. 7800mhz is stable speed for me.