PC only boots after i press down the power button when the psu has no power

[oz0]

Let me try to explain this correctly:
The pc powers up, but doesnt POST, all the fans are running and the DVDRW makes noises as well as the HDD, but no signal in the monitor.

Only after i shut down entirely the pc (by pressing down the power button 5 secs and then flipping the psu switch), if i press the power button to start the pc when its off, the lights blink and of course it doesnt power up. BUT then i flip again the psu switch and (it doesnt work 10/10 but its the only way to get it powered up).

After its on, it can be on for days... Never shuts down

I have some skills involving repairing PCs, has anybody have a clue about whats going on? maybe it can be fixed

Thanks in advanced for your help.

My rig:
e8400, Asus p5k-e wifi, 4gb Gskill 800 CL4, visiontek 4870, Maxtor 500GB 32mb, DVDRW SONY, Thermaltake soprano case and finally the ANTEC TruePower Trio 650w

 

[HOOfan 1]

Try replacing the CMOS battery. It is also possibly bad RAM or your motherboard is going on the fritz. A $4.00 CMOS battery replacement should be a fairly cheap troubleshoot attempt though.

 

[oz0]

Try replacing the CMOS battery. It is also possibly bad RAM or your motherboard is going on the fritz. A $4.00 CMOS battery replacement should be a fairly cheap troubleshoot attempt though.

Thanks, i'll try that. You're saying that the psu is perfectly fine?
I forgot to say in the first post that i tried my psu with another working pc (same hardware, friend of mine got same rig as me) and it did the same thing it does on mine...

 

[HOOfan 1]

It could be the PSU certainly...maybe there is a short in the PSU and the short circuit protection is shutting it down.