No matter how many cards this MB can support, it is just a bad mobo when it comes to stability. Take any algo, the hashes will move up and down, never settle. Z270 is so far the best, I have used.
Can u please gave me some more info regarding your rig configuration and settings:
BIOS version of MBO,
How much GPUs you have and what are settings regarding PL, Memory or Core cloaks of cards have you set,
Number of PSU and which one is powering what exactly,
When you say hashes will move up and down, do you think on pool side or miner side?