Interesting note:
When the R9 and R7 series came out from Asus, a number of Direct CUII video cards has major heating problems to begin with. Quite a few would get unstable and crash. When I was doing my research, I noticed that Asus quickly reissued new versions of their Direct CUII video cards shortly after release with a very slight model number change (so slight most people won't notice).
These new versions would run at higher (and louder) fan speeds by default. Interestingly, many of the Direct CUII video cards in reviews were actually these older (unstable part numbers) that had quieter fans.
I wonder if maybe the 7790 suffers from this same problem the newer R9 series had from Asus.
Well maybe I found the answer. Many people say that you must play with V-Sync otherwise the game decides to push everything to the max, which isn't unbelievable because my CPU and GPU at the time were both 100% Usage.
Solution?
Can you still do an RMA?
Try tweaking your fan settings so they are much higher speed by default (at all times). I think this was what they changed on the newer series, when the released the new updated models (aka I think they made the fans run higher, but I'm not entirely sure).