Having some trouble with a new build: built my new system and had it up and running just fine, but after 45 mins the system abruptly shut off. I wasn't doing anything too intensive (downloading new drivers in the background while surfing the internet) and charging my phone in one of the front USB 3.0 ports. Pressing the power button would allow the power button LED to flick on then off (just once; if you blink you'd miss it).
I opened the case and unplugged the power from the video card, and now everything appears to work fine. Monitor was plugged into the graphics card, rather than the MOBO DVI spot.
Here is the build; is it possible I overloaded the PSU and it shut down?
CPU: Intel i5 3570k
MOBO: Asus P8Z77-V LE Plus
GPU: EVGA GeForce GTX550 Ti
RAM: G.Skill Ripjaws (2x4GB)
SSD: Crucial M4 128 GB
PSU: Rosewill Hive 550w
Case: Cooler Master HAF XM
Since your PC reboots with the GPU plugged in but works without it, I would try booting with a different PSU.
Your hardware uses 300W at most so overload is not the issue.
For the HSF install, this is easy to check: start HWMonitor or similar SMBus monitoring tool and if your CPU cores stay under 75C under load, your HSF is installed good enough. If the temperature goes beyond 85C, you should consider re-seating it.
If the HSF checks out ok and the PC still reboots, I would try swapping out the PSU. Even the best models from the best brands have lemons/duds too.
HSF is seated properly; this thing will not start if I have power plugged into the GFX card. Unplugged everything seems to work fine. Could I be looking at a bad GFX card, or does this sound more like a bad PSU/cable?
Only two ways to go about it:
1- test your suspected parts in a known-good system
2- try known-good parts in your system
If a part you swapped in the known-good system cause it to fail, it likely has a problem. If your system still fails after swapping a suspected part with a known-good one, you likely swapped the wrong part.