PC cutting out after a few seconds - ATI 295x2 or PSU?

Hi,

I am having problems with my PC, and do not know whether it is the motherboard, graphics card or power supply. I can boot to windows with no problems, but after a minute or so, the PC cuts out and the graphics card fan maxes out. Screen blanks, no BSOD, lights on case remain on. Windows event log shows a kernel power critical event, details below.

I have tried the following:
BIOS update
Driver update
Graphics card BIOS flash (using on board switch)
BIOS reset to defaults
Memtest (All OK)
Cannot run stress tests as PC crashes before I can run anything.

The system runs fine without the graphics card plugged in via the onboard graphics. CPU/memory stress tests run fine. Changing PCI socket gives the same problem. I have replaced graphics card with a 560 Ti and it runs fine.

System has been running fine up until now for about a year, no changes were made prior to problem, first occurred during long stress test on CPU/GPUs.

I don't have an alternate PSU that can support the whole PC (current PSU is 1250w which should be more than enough). No over clocks on anything, temps seem fine prior to crash (maxing at 60C load) and fan on graphics card is running fine, pump seems to be working on graphics card too. I have a good cooling set up for everything with a H100 on the CPU which keeps CPU below 60C on load.

Any ideas? PSU is out of warranty and would rather not buy a new one, especially if it turns out to be MB/ graphics card problems as they are both in warranty.

Thanks,

Matt

Details:

> Log Name: System
> Source: Microsoft-Windows-Kernel-Power
> Date: 09/09/2015 18:21:29
> Event ID: 41
> Task Category: (63)
> Level: Critical
> Keywords: (2)
> User: SYSTEM
> Computer: Broze
> Description:
> The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.
>
> DXDiag:
>
> ------------------
> System Information
> ------------------
> Time of this report: 9/9/2015, 18:46:31
> Machine name: BROZE
> Operating System: Windows 8.1 Pro 64-bit (6.3, Build 9600) (9600.winblue_r9.150322-1500)
> Language: English (Regional Setting: English)
> System Manufacturer: ASUS
> System Model: All Series
> BIOS: 2604
> Processor: Intel(R) Core(TM) i5-4690K CPU @ 3.50GHz (4 CPUs), 3.5GHz
> Memory: 8192MB RAM
> Available OS Memory: 8068MB RAM
> Page File: 2562MB used, 9506MB available
> Windows Dir: C:\Windows
> DirectX Version: DirectX 11
> DX Setup Parameters: Not found
> User DPI Setting: Using System DPI
> System DPI Setting: 96 DPI (100 percent)
> DWM DPI Scaling: Disabled
> DxDiag Version: 6.03.9600.17415 64bit Unicode
> Support Details:
> Motherboard: Asus z97-K
> Motherboard Driver:
> Motherboard Chipset: Z97
> Motherboard BIOS: 2604
> CPU: i5 4690K
> Memory: Avexir 8Gb DDR3 1600
> Memory Latency: 800Mhz
> PSU: Seasonic X-Series 1250w '80 Plus Gold' Modular Power Supply
> Graphics: Sapphire AMD Radeon R9 295 X2 8192MB
> Graphics Driver: 15.4
> Sound Card: n/a
> Sound Driver:
> Operating system: Windows 8.1 pro
> Changes prior to problem: N/A
>
>