Humm... it's bad to say this, but in most Mobos that changed to Legacy state problem was solved. The problem is
still tied with Elpida Memory.
The two last things i can ask you:
* To test the card on another machine(that haves a PSU that can provide 42A at the 12v rails
like your PSU do).
Stress the machine on this other rig!;
** To check if your BIOS is updated to the latest version(BIOS V1301). Many new manufacturers updated their BIOS in order to support better the R9 290(x).
When the above two things were done and problem not found, i will suggest you to go to
ROG Forums and ask them about the problem. Here you will find people that know more than us about the ASUS hardware.
I will ask you about contacting the ASUS official technicians on the forum and ask them about a BIOS or vBIOS update for solve this problem.