I had exactly same problem when I crossfired 5770, I was getting unexpected atikmdag.sys error anytime in Windows 7. I googled and found somewhere that atikmdag.sys error in crossfired GPUs often occur due to two ATI/AMD HD Audio devices in Device Manager. Disabling one of them may solve this problem permanantly. This has solved my problem of atikmdag.sys. This is obviously a driver bug which has not been solved yet.
I don't know if it will work on your 7970, but at least you do can try this. If you see two ATI/AMD HD Audio Device in device manager, disable one of them and restart system and wait to see it it crashes again or not. If it crashes again then do vice verca.
Good advice my friend