System randomly rebooting when under load

Hi all,

About two weeks ago I upgraded my GPU from an Asus GTX550Ti to an MSI Radeon R9 290 Gaming 4G.

Since then I've been having random system reboots when gaming or doing other menial tasks. These reboots happen at seemingly any time the system's power consumption goes up, though I cannot confirm.
Also, when I power the system down via the Windows "Shut Down" start menu option, the PSU, front LEDs, and mobo stay on despite the system being "off".

Some detail on reboot instances
* Started up GPU stress test software, let it run for 5 minutes or so and all seemed fine. Temps were good, nothing looked weird. I then alt-tabbed into Chrome Canary and opened a new tab, which immediately caused a system reboot.
* Started up Diablo 3, played in a party of four, ran fine for an hour or so, then system rebooted when there was suddenly a lot of on-screen activity
* Started PC, system idle with Chrome Canary open along with other hardware monitoring software, system just rebooted randomly

Looking at this, would it be at all possible that Chrome Canary is causing a system reboot? I know that Chrome is heavy on memory and Canary is an unstable build, though I don't see it causing system reboots. I've never had such issues with it before.

I've taken the following steps to solve the issue:

* Swapped out old 600W PSU for a GIGABYTE ODIN GE-S800A-D1 800W (second hand from work)
* Reseated CPU, applied new thermal paste, reseated CPU cooler ( cooler is aftermarket cooler )
* Checked all mobo and GPU connections countless times
* Removed all overclocking in the BIOS
* I had to update the mobo BIOS for it to work with the card
* Removed each stick of memory individually and re-tested system
* Tried running the GPU from a separate 600W Power Supply
* Only tried once, have since stayed with the Odin
* Reseated mobo in chassis


System information:

Operating System

* Windows 8.1
* Do have other OSes installed on Hard Drive, use GRUB as bootloader

CPU

* Intel Core i5 2500k
* Runs at ~40C idle
* Runs at ~60C under normal load
* Runs at ~70C under heavy load
* Turbo Mode is enabled, set to 3700MHz in BIOS

Mobo

* Asus P8Z68-M PRO
* Runs at ~40C idle
* Runs at ~50C under load

GPU

* MSI Radeon R9 290 Gaming 4G
* Second hand, previous owner used it only for gaming, reports no issues or such system reboots while he owned it. Previous owner is trustworthy
* Runs at ~75C under normal load
* Runs at ~90C under heavy load

PSU

* GIGABYTE ODIN GE-S800A-D1 800W
* Second hand, was used at work, bit dusty when I got it, still had cable ties so I assume it wasn't used much
* Sometimes runs quite hot though not normally
* Both red PCIE chords being used
* 8-pin on GPU
* 6-pin on GPU
* One chord goes to mobo power
* One chord goes to mobo 8-pin
* One chord powers 3x hard drives
* 2x 500GB Seagate
* 1x 80GB Seagate

Memory

* 1x Kingston HyperX 4GB
* 2x 2GB Corsair ( I think ) MX something something

Any help or insights would be very much appreciated.

Thank you. I'll try that tonight. For now, assuming that it still reboots, might you have any other ideas?

Aye, that's the first time I've ever done that. I wouldn't have if the GPU worked fine without it . I'm looking to get a new mobo, though no use spending a lot of money if I have one that works fine. The system pretty much just goes straight to a black screen and starts back up again with no warning. No blue screen or information on the screen - it's as if you turned the wall plug off.

An update:

System reboots when GPU is under high loads. Problem persists with two power supplies. Could this be the motherboard's fault, then? Seems to be the only constant along with the GPU.

Thank you. I'll look into replacing the replacement and see how that goes . I will update you once I have done that. Do you think that it could be that the motherboard is unable to deliver the power the system needs?

The PSU was the problem. I swapped the 800W Odin out for a Corsair RM750 and I've yet to encounter such a crash. Thank you all very much for your help and patience!