Maybe you lost in the silicon lottery? Each chip performs differently to overclocking. You might of been extremely unlucky and gotten two duds. You can try adding some voltage to help stabilise some clocks. I currently have two 7850's running at 1200 with no voltage changes -- I was lucky.
It takes time to overclock, and do it slowly. Go up in 25-30 mhz increments. If its unstable, step back ten. Thats how I do it. Up 25, down 10. If you get to the max overclock on that voltage, if you want to, you can add more and proceed further. It takes time to overclock. It took me at least a week of 4hrs+ a day of stress testing to get where I am today.