Computer randomly turns off

I have a problem with my computer shutting down randomly. it isn't a windows shutdown and happens without warning. i want to know what is wrong if someone could help that would be brilliant.

computer
AMD Athlon 7550 Dual-Core Processor x2
N68C-S UCC
NVIDIA GeForce GT 430
DDR PC2-6400 2gb
psu: is un labeled but says to be 500 watts, and 18a on 12v1 and 16a on 12v2

This is what i know about the problem
-it probably isn't software related, i have run anti-virus (avg, microsoft security essentials, malware bytes), also it can sometimes happen before windows starts, or even before i get a single beeb from my motherboard speaker
-its not my temps, i have run core temp and it doesn't seem to happen when the core temp is high (even then my load is about 50 degrees at most after 6 hours of prime95)
-seems to be more often during startup, or relatively soon after startup maybe 30min) it happens much less if the computer is doing a task, only once happend during a game, and once during prime 95, (after 6 hours)
-i have moved by ram from one slot to the other, doesn't seem to make a huge difference
-started after september 2012 ish
-i don't think it is a sticky case power button, i changed the reset button so it is the power button, didn't really help (now changed it back)
-it seems to happen much more often on the internet, particularly youtube
-my motherboard doesn't seem to have anything wrong with it, like leaking capactiors (though i am not at all an expert)
-i don't have any ram or psu hanging around the place to switch it with
-the computer doesn't ever warn me it is going to suddenly going to crash
-it happened once during bios (though can't exactly remember)

any tips, i also would prefer if multiple people gave an answer just becuase then i have multiple opinions(i might add to this later on if i notice any more pattersn)

also is this in the right place, i didn't know where to put it

Thanks in advance