hello guys,
this week i bought 6 gpus XFX 480 8GB 1288 Mhz. i flashed all to 2080Mhz VRAM and change timing setting from 1775 to 2000Mhz .
2 of this 6 cards freeze on grey or black screen after 2-5 minutes, sometimes shows up artefacts. i change windows, amd drivers and claymore versions and  the problem still exists.
after that i take one of this card to test on another computer, i put it on pci-e x16 slot to test. on benchmark there isnt a problem. only when i start mining.
after that i change bios switch to default, without deleting drivers, the memory clock change to 2000 Mhz and performance change to 22-23 Mhz in claymore. and again freeze on grey or black screen after a couple of minutes. all pc crash until restart. i found that two of this cards are version 1.5. another cards are 2.3.
any idea what is the problem and where i make a mistake.
the benchmark on gaming there isnt a problem. are these cards has defective memory.