Sapphire rx 480 8 gb

hi all
i`ve 6 cards sapphire rx 480 8gb ddr5 nitro oc
actually they were 2 types
3 cards have a 512 kb bios
3 cards gave a 256 kb bios

i`ve flashed the all 6 cards

after that i changed the Memory Clock only to 2200 and left all other settings to defaults
the 3 cards which have a 512 kb bios working with no problem
the 3 cards wich have a 256 kb bios if i try to change the memory clock to any value over 2000 i always get (GPU0 0.00 mh/s) in the claymor`s mining tool

so i`m right now working on 3 cards with memory 2200 and 3 cards 2000

if there a good settings for overclocking that card for best hash rate hope to share it with me coz i`m new

i have a 8 gb ram
i`ve 50 gb virtual ram
working windows 10
driver crimson 16.9.2