Power problems with first build

Hi i recently build a computer. these are all the parts.

Asus p8z68-v gen3 motherboard
Corairs 650 tx power supply
Gtx 560 ti superclocked graphic card
intel i5 2500k processor
8G of ram
cooler master 212 plus cpu fan
Windows 7 ultimate (64bit)

OK now on to my problem. Every time i shut of my computer and it lasts a few hours off, i need to flip the power supply switch on the back for a few seconds to be able to turn it back on. I don't know what is causing this. I don't know if it is a setting on the bios or something wrong with the power supply. It is not a big problem but it gets kind of annoying after a while. Any help would be appreciated. PLEASE!!
 
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