Computer wont start...kinda

[mythicalmoose]

Since I built my system I have always had problems booting it. If I just press the power button it will briefly start up (fans spin, HDD and optical drives lght up) but turns off again after about 2-3 seconds. If i hold the reset button at the same time, release the button for 2 second and hold it in again for about 10 seconds it will finally boot. Any ideas on what the problem is?

MB: gigabyte GA-MA790FX-DQ6
processor: AMD phenom 9600
video: ati radeon 3870
PSU: ultra 600 watt atx
case: coolermaster centurion 5

 

[Lexluethar]

Power cables coming from the case connecting to the motherboard may be connected wrong. Double check those.

 

[Naphtali14]

Ya, triple check all the cables.. Is it a new power supply?