Thank you for the follow up information it's incredibly helpful. I'm using ethOS and haven't put these cards in a Windows box, but will try and tweak the best I can through BIOS mods and the ethOS config.
Are you flashing the card to 1191 and 2191 or are you using a config to manage those more granular tweaks?
You may be right about the card starving for power, I have them all set with a ptune of 3. Do you set a ptune value or let the new timings decide?
RE: Basic Guide: Mining Ethereum with RX400/RX500 GPUs