PC shutting down when switched on

[sgj320]

Hi a month ago I built a system with the following spec

Athlon phenom 8450
2gb ram ddr2
Nvidia GTX280
850w power supply
dvd and dvd burner
1x case fan

When I first put the system together it had problems switching on but then trying few times it switched on. Occasionally when I turn it on it switch itself off then on again second later. But the system seems stable. When I'm playing fear2 it doesn't freeze up.

Could it be a dodgy power supply or could it be am pushing it too hard? Asus says my comp needs 700watts power. I used their recommended wattage calculator.

 

[samuraiblade]

what make is your 850W psu , id lean towards that depending on who made it , also your motherboard make and model

 

[sgj320]

it's some crappy make called silver power. I couldn't afford a tagan. The motherboard is an asus m3n-HT