At the utter limit of what the motherboard can supply. If you fully overclock it at stock volts (for me at least) it will trip the motherboard and shut off the system.
I think it's the total draw (tested with 150W and 180W bricks with the same results) causing the issue as disabling two CPU cores reduced the restarting but obviously is not the best when you want to use the system.
This may be a limitation of the 560M version, 570M versions came with different power components so may be up to the task.
However with a slight (1.05v to 1.025v) undervolt I have gone from stock speeds of 850/1200 to 940/1575 which is a nice level of performance to have.
GPU power rework + pads for back of gfx cards.
Pic of it all installed.
Pic of modified 5870 heatsink.
Pic of the card
985/1575 at stock volts, note that it's not quite stable at these settings (will shut off under peak load).
I think I have zeroed in on 940/1575 1.025v for 24/7.