sounds really weird...
something i would try
-check if mainboardbios settings are right
4g decoding on, pcie = gen 1 all
disable everything you dont need
-ddu + install new driver with device manager
for later than blockchain driver
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0002
add dword KMD_EnableInternalLargePage=2 in registry for gpu workloadmode
disable crossfire and ulps in registry
EnableCrossFireAutoLink=0
EnableUlps=0
maybe you fucked something up with bios?
reflash stock bios and recheck
are your risers alright? connect with 4 or 6pin never with sata, you wouldnt believe how many problems risers can cause
the brand is not the problem im pretty sure, i have everything mixed up...580´s/vegas from all brands working fine

Okay thanks I'll try that out once I get home thank you! Yeah the risers are powered through Sata.. I'll switch it to periph and see if that changes anything. I don't know if I fucked up the BIOS.. if the HDMI is plugged into the MSI card it mines at 30 mh/s but I'll run a DDU on it and retry. If not I might just give up and wait for a decent priced Sapphire in stock.