PC doesn't boot, no beep

Hi,

I've been having an issue with my PC for a while now in that sometimes when I go to start it, it doesn't boot. I press the power button, the fans start up but then there is no POST beep, no display and after about 10 seconds the PC switches itself off. It then turns itself on again a few seconds later and repeats this process of not booting until I manually turn it off. This problem doesn't happen every time I switch it on and will normally turn on as normal 10-15 minutes after initially trying. I've tried reconnecting all cables and components but it hasn't made a difference. It's tricky to check individual components as the problem doesn't occur every time I try and boot.

Here's my set up:

Intel Core i5 2500
Gigabyte GA-H67M-D2
450W Corsair CMPSU-450VXUK VX
Corsair 8GB (2x4GB) DDR3 1333MHz XMS3 Memory Kit CL9 1.5V unbuffered

Can anyone give me some guidance?

Thanks.