Unusual boot problem

Hi Folks,

I built a PC about a year ago, and it has been working fine during that time. No problems at all.
But during the last few days, I'm getting occasional boot problems.

When I power on the pc, sometimes it will boot immediately, but other times it takes 5 - 10 seconds from pressing the button 'till it boots up.

Same with a mouse click to wake from sleep.

One a few occasions, it won't boot at all, and then after a few minutes, it powers up.

Pressing the power button on the mobo doesn't help, nor does the reset switch.

I'm getting power lights on the mobo and gfx card, I've tried reseating all the components, but I'm still getting this occasional problem.

Any ideas on what it might be?

Cheers

Shinobi XL case
Gigabyte Z77 UD5H mobo
Asus 7970 gfx
Corsair Vengeance Ram
Corsair AX750 psu
Crucial M4 SSD
i5 3570k