It isn't the PSU. I would try some other drivers for your GPUs. 12.8 iirc was quite a decent set. I assume you have a temp monitor running and can cofirm heat isn't the issue?
Yeh my 7970s didn't go past 60 degrees. My CPU stayed a nice chilly 40-50 degrees. I had logs going so I could see all readings till the point of the crash.
Arent we only upto 12.11? So 12.8? Me=ConfuseD?