Please help computer wont turn on (sometimes)

[jagsdd]

Hello,
i recently built my own computer, the components are as follows:
CPU: AMD x4 ii 955
PSU: cit 750watt
GPU: ati saphire radeon 5570 1gb ddr3
Motherboard: ASrock 870 extreme 3
RAM: 16GB corsair vengeance
HDD: Seagate Barracuda 1.5tb sata III(but i use sata II cable)
OS: windows 7 ultimate 64bit

on first boot everything was fine, it worked well and still does but the problem is turning it on. sometimes when i click the power button it doesn't turn on (the fans spin a little and LED's turn on for a very short time then everything stops, if i then click the power button again nothing at all happens. to overcome this problem i have to unplug 24pin and 8pin power from motherboard and turn off the PSU from the back, i then wait a little, turn the PSU on plut the 24 and 8pin to the motherboard and then press the power button, even this does not work sometimes. i have replaced the PSU already, the power button is also fine and is not stuck so is the reset button. i cant work out the problem with my computer, if anybody know what the problem is please help. thank you in advance. By the way I'm new to this site.

 

[Novuake]

Ok I have seen this before. Once it was a RAM issue, once a problem with compatibility, all the other times it was PSU.

Try booting with just one RAM chip plugged in and see if it improves.
Your RAM should be compatible with the motherboard, so I will not suggest that.

 

[jsc]

Try a known good PSU.