i wouldnt apply strap to allready modified bios, you have choice use modified bios or stock bios with claymores strap.
flash your cards to stock bios then try again,and also dont forget virtual memory set high as you can.
Claymore -strap will override whatever memory timings are set in BIOS. It does not matter if you have stock timing or have flashed a modified BIOS. Restoring to original BIOS is nice if you sometimes use the GPU for gaming or want to have the card ready for sale, but makes no difference for mining when you use the -strap option in Claymore.
True, proven that this is so, I have done several tests lately, no matter how to use stock bios or modified bios, -strap applies the same.