that should be way more than enough power for that system, since the draw should only be around 250-275 watts (you could have gotten by with a 450w that was 80-plus certified ).
there aren't many ways to check if it's a motherboard or not.
do you have a spare hard drive sitting around? try disconnecting your HDDs on your system - all of them, if you have more than one - and installing some operating system onto it. see if it causes freakout errors still. that'll narrow it down to your hdd or motherboard. if your hdd is failing, dropping enough bits between the head and the system itself would cause weird freezes. if that's not it, try another motherboard.