[jqp]

I just replaced an Antec SmartPower 350, that came with my case, with an Antec EA500.

The reason: when I shut down the PC, the PC would not stay off, but restarted - I had to switch the PS off with the switch in the back. To power up the PC I had to switch on the PS, wait a few seconds, then push the power button, two or three times - it would then continue to power up.

I figure there are a few possibilities:
1) power supply was over-taxed by draw of all the drives I have in the system
2) power supply was damaged (haven't tested in a new setup)
3) some loose connection

Any ideas?

[Levi]

I am not sure if it is a powersupply problem specially if the unit will not completely turn it self off. 

Since powersupply are inexpensive, I would replace the unit.  Buy from a reputable company just incase it is not the powersupply.

My guess is it is the operating system.  I've seen this with Windows 98 machines and some Windows XP. 

Here are signs of failing powersupply:

Computer devices attached to the powersupply behaves irregularly [some devices seems to work but other devices works some of the time]
Over heating will cause your computer to just shut down completely or reboots spontaneously.  The same will happen when it is over taxed.
etc...etc...

Good luck.