@supertux1
I did nearly the same as you. use v4 for all cards and and lower the power consumption with watttool.
I use the pool ethermine.org and at least clay 9.3 reports 0 rejected. For dcr i am on suprnova with ca. 1.5 % rejected shares.
Yeah just one of my cards can't take v4 for whatever reason and I verified it stand-alone being the only card in the system, fan %100 and
no volt drop to eliminate heat and lack of power as a cause - still fails. It takes the other bios just fine. (Lower quality RAM? Who knows.)
I tried testing the miner with just one card and only mining ethereum to eliminate any power drop or dual mining interference as a cause
and I still get quite a high percentage of rejected shares. (good cards and crappy card alike.)
My next options are to try a different mining pool, try the stock bios, try the older non-patched driver, try without WattTool settings etc...
I think what I might do if the simple fixes don't yield any results is to make my own bios.
That would be taking stock (non-oc) bios which runs the CPU @ 1260Mhz and copying the just the ram timing from 1750 to 2000.
Then I'll see if that's stable. If it is, I'll use WattTool to change everything else.
FYI I used to run my main monitor off of the VGA port built into the motherboard, but I found that WattTool will not work unless a display is plugged into one of the cards. Sort of like how AMD Settings won't run unless one is hooked up. I think the driver does something to itself when it detects something.
(That frustrated me to no end and paid like 75 cents a day in extra power costs till I figured it out.) I have ordered an HDMI dummy plug.