lbr, I'm having a similar problem on Windows 7 Home Premium. I have 6x XFX 7970s on a Gigabyte 990FXA-UD3 with both x1 slots pin shorted. I have tried 14.4 (moded inf files), 14.6 may 23 (app), and 14.7 july 9 (app). What happens is I either get code 43 on 1 out of 6 gpu's (it's a different gpu with each catalyst version), or my computer instantly shuts down between the windows logo screen and the Welcome screen, until I do a cold reboot, then I get code 43. I think it may be a Windows problem because I'm able to run BAMT on this rig with the 14.6 linux drivers (though I only get HW errors because I can't figure out how to install sgminer in linux properly). I've been able to install 14.6 on four 6 or 7 gpu rigs with Windows, including one rig with the same motherboard and 6x r9 270X's.
Have you tried 13.12? Or even 13.1 from techpowerup?
The problem is that 'code 43' occurs not only when driver are not configured properly(this scenario is solved by mod), but also when something else is wrong..
Whats the revision of your 990fxa-ud3?
I'm running 6x290 on x1 risers on 990fxa-ud3 rev:3.0. However I helped a guy setup 6x290 with same mobo but 4.0 revision and it din't work. Nor in Windows nor in BAMT nor in xUbuntu. Windows reported code 43, *nix hanged on boot. So there is something wrong with this mobo prly. The issue was solved by swapping one 290 with 7950..
Also, that 6x290 on 990fxa-ud3 refused to boot at all headless untill I set "Full Screen LOGO Show" to "Enabled" in BIOS - very odd.
Tho.. if u say BAMT works.. then there is deffo something wrong with Win.
Anyway, pm me TV id/pwd, maybe I will be able to find out whats going on there.
Thank you for the offer to help me personally, but I feel that you are far too talented to waste your time on me. I just tried 13.12 with the same results. Both the rigs with 6 gpu's run rev 4.0 of the 990FXA-UD3. One works one doesn't. I've tried swapping in a 7950, but that didn't help. I've been properly cleaning up the drivers with one of your old instruction sets. Aside from the type of GPU's, there are only 3 other differences in the rigs: (1) this rig has 4gb of ram; the working one has 8; (2) This one has dual identical 1000 watt psu's; the working one is a single 1250 watt psu; and (3) this one is running Windows 7 Home Premium installed and activated by the previous owner; the working one is running windows 8.1. I'm thinking the last bit is the most important. Since Linux works, I really don't think it is a hardware issue. I'm going to try and borrow a hard drive and put a clean windows 8.1 install and the 14.6 may 23 drivers and will report back. I think this is an isolated case rather than a bug with your software.