Hello!

I'd suggest trying playing with different switches in cgminer. I have 2 280xs which are basically 7970s. Here is what you should try: --gpu-engine 1098,1084 --gpu-memclock 1500,1500 --thread-concurrency 11200 -I 13. First i set gpu-memclock to 1500 which helped me to get above 600 then, thread concurrency, then set gpu-engine clock and try a few values between 1050-1100, and wait a few seconds for the hashrate to stablize. Hope this help, i  don't think flashing new bios will help. Also try it on a fresh windows, it helped me a lot too.