I switched from sgminer-gm 5.5.5 to xmr-stak 2.2.0 and tweaked amd.txt as suggested by @Vlizzjeffrey and I'm getting 1370 H/s out of (3) 2GB XFX RX 560 cards. That's exactly the same as with sgminer, but it seems a lot more stable. The specific problem I was having with sgminer is it would lock the rig up so bad it needed to be hard reset, usually, but not always, when I used the KVM switch to select it after it had been running awhile. Note that I am using the integrated GPU in the Celeron CPU for video, and not one of the mining cards, so this behavior was quite perplexing.