I doubt it is an issue related to PSU as I play other highly graphics intensive games like BF3, crysis 2 and even shogun 2 before FotS DLC with my HD7870 smoothly and without any problems yet.
I even tried to run furmark for more than an hour without any issue so far. Furmark draws much more wattage than any other applications/games.
So i doubt it is related to PSU issue. So far seems all the people who came across this freeze bug are using Radeon HD7XXX series cards, even run on low setting and uncheck all the graphics settings still has this issue.