PC auto restarts/shuts down on boot

[wassup99]

Hello,
I built my own PC on Monday (bad idea), installed Windows 7 on it etc. and it's working fine. Tuesday I come back from work, working fine, installed a bunch of Windows updates at shut down.

Wednesday I come home and my PC starts shutting down by itself on bootup. First it'll reach Windows, work for 2-3 minutes then just off, like a black out like someone cut the power. I try to turn it on again but it repeatedly cuts itself off at faster and faster intervals. Thinking it's a heat problem, I wait for a while and boot up again, checking CPU temp in BIOs, shows 47C. Reach desktop, run speedfan, shows 38-42C for all 4 cores. PC runs fine the rest of the night.

Thursday night, same problem. Runs in windows for 2 minutes, off. Then offs itself in 1 min, then 30 seconds, before I can even reach BIOs. I let it rest for another 10 minutes with the side casing open, boot up again, CPU temp 47 C, speedfan 38-42 C, now it's been working fine again. I've run Prime95 before and it goes up to 50+ C, but even 50 C shouldn't cause a power cut.

Either a bad mobo or PSU - how do I test?

MSI H67MA-E45 Mobo
i5-2400 3.1GHz
Coolermaster Hyper 212 Plus
Seasonic 620W M12II Bronze Modular
Sapphire HD6950 2GB Dirt3
4x4GB Kingston 1333 DDR3

 

[henydiah]

Try use vga onboard , uninstall driver vga, unplug ac from psu, remove hd6950.
Check all cable conection also cable HDD or try use another cable HDD. Also posible to install fresh os bvy formating hdd.

 

[wassup99]

lo and behold i unplugged the power to my gfx card and it works perfectly now on boot up. i can hear my fans operating at higher speed though, as though previously there was insufficient power to run everything. But does this mean my PSU sucks? should i get a new one? I thought Seasonic 620W is a really good PSU