So, I have several B250 that I have the same problem with as well. Primarily, the CPU soft lockup garbage. I tried another Linux mining OS and it did the same thing.
The hash rate drop is interesting. I have one rig with all Nvidia cards that I didn’t have the problem with. The other two are AMD and I have the problem with. On one I found if I backed down the OC settings it stabilized. The last one I have to go back to the farm and power it off because the CPU soft lockup has the CPU pegged and it’s not responding to Hive reboot commands.
I found that running all PCIe settings in Gen 2 seem to work the best. I also found that when I’m getting the amdgpu driver errors or CPU soft lockup if I do a hard power cycle (sometimes a couple of times) it’ll come up clean.
I am sure that it’s something to do with this board and Linux. From my research the CPU soft lockup has been attributed to power to the CPU. I don’t know that this is true because I’ve got 2 x Corsair 1500w platinum PSU and it will do this with or without cards plugged in.
Anyway, I’m looking into different CPU settings in the BIOS trying to see if any of them make a difference. The latest BIOS update as of 07/20/2018 didn’t fix the issue, FYI.
Let me know if you figure something out and I’ll do the same.