First of all, you didn't inform us which driver versions you are using. Which PBE you are using for modified your bios? I suggest using PBE 1.6.2 instead 1.6.6, PBE 1.6.2 will copy strap 1750 to above (more stable based on my experience). And don't forget to use Crimson ReLive Edition Beta for Blockchain Compute driver. And the last thing, patch your driver signature, so the system will recognize your modified bios.