At 6400 I've run memtest86 for 24hrs and the memory reports no errors so the issues with the bios doesn't seem to be memory related. I've tried running it without XMP and I still have problems.
I've narrowed the audio/video stuttering specifically to the use of the default "auto" setting in the fan type when a manual fan profile is used.
If I use a manual fan profile (controlling my pump speed and setting the fan speeds dependant on water temperature) with the fan type set to "auto" i get the audio buzz and video stutter. If I set the fan type to the exact fan type (PWM or Vol) the the problem goes away.
This bios is unstable though. The motherboard doesn't always sleep or shutdown properly needing a complete power down to recover. I can tell when this state is present because the onboard LEDs stay illuminated when the system is off despite the bios setting in bios for LEDs Off when powered down.
Sometimes on boot the network isn't detected. This is only solved if I go into bios and use F10 to exit without making any changes.
Sometimes the system won't boot with a windows BSOD reporting ACPI BIOS Error
Gigabyte say I can regress the bios back to an earlier version but I'm nervous about this because I've heard of others bricking their system trying it.
I'm beginning to wonder if the motherboard itself is suspect.