Wow, that sounds very strange. Do you have other ref V56s in the other rigs using the same timings? If so, what does the auto-tuning choose for those gpus?
i have other rigs with vega56 ref (sams mem) but different brends, they all use lucky timings from trm help file
on one rig autotune sets 14*13:AAA and on another 14*14:AAA but all of my vegas56 (expect those x3 that i describe before) gives average 2230-2300 h/s on cnr
p.s. may be i don't know it's problem from motherboard?! i have h110 asrock btc pro (13 pci-e) and celeron 2.8ghz, 4gb ram