Dunno if it's of any help to fix unstable hashrate but I settled on this config for rx570 4gb (880-910 per card).

{ "mode" : "GPU", "worksize" : 8, "alpha" : 64, "beta" : 16, "gamma" : 4, "delta" : 4, "epsilon" : 4, "zeta":4, "index" : 1, "multi_hash":448 },
     { "mode" : "GPU", "worksize" : 8, "alpha" : 64, "beta" : 16, "gamma" : 4, "delta" : 4, "epsilon" : 4, "zeta":4, "index" : 1, "multi_hash":448 },

And the same but multihash 992 gives me 1830-1870 from vega56. 11.1kh from 6 gpu rig. 1014watts. That rig has been working for 24 hours and hashrate drop was insignificant (10.9kh). But the other rx vega rig works best with multihash 960. I may be wrong but looks like both threads have to have equal multihash and that was not the case in the previous versions.

Big thanks for this great miner!