I have a mATX board lying around, I will try and test it tomorrow.
The second system doesn't have the powered riser, it is a regular 4x PCIe slot so max 25W, the rest is provided via the PCIe PEG connectors.
I've just sold my GTX970, but it didn't have these issues. It might be specific to the R9 Nano.
If you have a link to the article you mentioned that would be great.
The Toms Hardware review shows very low power consumption on the 3.3V rail, only 3W peak so I'm fairly confident that isn't causing the issue.
I know that Furmark is unrealistic, but I do think the system needs to be able to at least run it for a couple of minutes. the Toms Hardware review shows that because the way Powertune on the R9 Nano works that the power consumption is generally lower in Furmark then in a game.
I'm still baffled at the fact that some resolution work with Furmark and some don't.
For example:
Furmark 1080p 0xAA = Monitor goes blank
Furmark 1080p 2xAA = Monitor goes blank
Furmark 1080p 4xAA = Seems stable after 5 minutes (PowerLimit -50% - Fanspeed Fixed 50% - GPUTemp 37C - Core Clock ~430MHz - Core Voltage ~0.9145V)
Furmark 1080p 8xAA = Seems stable after 8.5 minutes (PowerLimit -50% - Fanspeed Fixed 50% - GPUTemp 38C - Core Clock ~540MHz - Core Voltage ~0.9325V)
Furmark 1080p 0xAA = Monitor goes blank
Furmark 1080p 2xAA = Monitor goes blank
Furmark 1080p 4xAA = Seems stable after 3 minutes (PowerLimit +50% - Fanspeed Fixed 50% - GPUTemp 70C - Core Clock ~908MHz - Core Voltage ~1.07V)
Furmark 1080p 8xAA = Seems stable after 6.5 minutes (PowerLimit +50% - Fanspeed Fixed 50% - GPUTemp 70C - Core Clock ~1000MHz - Core Voltage ~1.19V)
Heaven 1080P Ultra 0xAA = Seems Stable after 15 minutes (PowerLimit +50% - Fanspeed Fixed 50% - GPUTemp 69C - Core Clock ~1000MHz - Core Voltage ~1.19V)
I'm really starting to feel it has something to do with how the GPU is loaded.
Extremely light loads = fine
Extremely heavy loads = fine
moderate loads = fail
Although running Heaven on any setting doesn't cause it to crash.
I'm just not happy with 90% stable it needs to be 100%, especially because the instability happens in a weird way.
[USER=43]@Josh | NFC[/USER] it might be a bit early to say but you might be getting some questions over on [H] if the powersupply is this critical when people are using the R9 Nano, I didn't have these issues with the GTX970.
Edit:
Guys!
I'm so so so so sorry.
I pulled a PEBKAC 
You know 2 posts up I mentioned Heaven at 960x540 only running @ 5 FPS on the ASUS Q87T.
I've been using a program called Prio for years, it allows you to manage you task manager and save the affinity of processes. I had Heaven set to use only 1 CPU core -_-
