Random shutdowns then now no POST/black display - stumped!

[Mr.Pie]

Parts list

• i5 4690K
• 2x4GB Kingston DDR3-1866 CL9
• MSI R9 390 8GB
• Fractal Design Newton R3 1kW - going to expand to CFX this summer when I get my bonus

Current problems:
Currently rig is not posting/black screen. Previously started randomly rebooting when gaming.

Things I've done to troubleshoot

• When my rig was still working, I ran Heaven for approx 2hrs with no artifacts/crashing. Briefly ran Intel Burn test on max mem for 5 runs and all stable. Everything was returned to stock clocks/values and crashing would still occur after approx 1. hour.
• I've removed my R9 390 and also reset CMOS and tried to use the IGP - no dice. Still no post/black screen.
• Something else I also tried was to use a different power strip/wall socket but still no dice.

I'm absolutely stumped on where to start - any ideas on what to RMA first? I have no spare components to use/friends with spare parts.
This rig/setup has been rock solid for the past 7 months or so.

I initially planned to RMA my motherboard first - but no sure if thats what I should do first....

[shaithis]

Try with one stick of RAM at a time.

After that, it has to be PSU or motherboard. Random rebooting would make me think PSU ahead of motherboard...

[Mr.Pie]

Try with one stick of RAM at a time.

After that, it has to be PSU or motherboard. Random rebooting would make me think PSU ahead of motherboard...

Will do.

Yeah - since it was "Heaven" stable for a good 2 hours it shouldn't be the GPU. But it would only crash/random reboot after gaming for an hour or so. I didn't really do a stress test on the CPU other than 5 runs of intel burn test at max mem.

I'll give single RAM stick booting a go sometime this week. Do you reckon I should stick to the IGP rather than my R9 390? I've reset the CMOS but had no dice on display.