I bought an Aurora R4 just over 2 years ago, and I just put in two R9 290Xs. I haven't made any changes to the system besides replacing the graphics cards.
They work fine on their own not in crossfire, but playing games in crossfire causes increasing lag as they heat up, and cause the system to shutdown without a blue screen and then boot back up.
The rebooting does not seem to be linked directly to GPU temperature, since it reboots sometimes right after loading a game.
Is the stock 875 Watt PSU not enough?
I recommend you try a clean installation of the video drivers as explained in this article and test a different Crossfire cable.
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