Since your power supply is brand new, and rated above the recommended wattage, then that means that a too-small supply may not be the immediate cause of your problem.
Keep in mind that does not change anything the other posters are saying. I agree with them completely - a borderline PS is not a good idea. I'm just saying that, for now, that may be a separate issue.
The issue for now may be that the power supply is defective - meaning it is intermittently not putting out all of it's rated power.
But it could be other things too.
A lot happens when you push the button on your computer and before you see that first BIOS screen. Your issue could be due to anything that happens during that 2 seconds of time that could interrupt the process to begin the BIOS startup.
When you press that power button it simply closes a circuit between the case button and the PWR SW header on the motherboard (2 wires - they simply contact together).
So the first point of error is that switch. But your switch seems to work fine because you get to step 2:
When that PWR SW circuit is closed it basically sends a signal along a "logic circuit" on the motherboard to the Power Supply telling it to switch from a low-power "standby" mode to the "full power mode".
So the second point of error is that logic circuit. Any defect and the power supply will not switch on.
When the power supply is switched on that activates the BIOS which then performs a POST routine (Power On Self Test). The routine checks the CPU, the RAM, and the System Bus, (and more) for connectivity and compares what it finds against it's stored "memory" of what should be there. If all checks out you will normally get a single BEEP from the BIOS - the POST Beep. Unfortunately, many modern systems have eliminated the POST Beep - yours may be one of them.
That's the third point of error. If the POST does not complete the boot process will stop. This indicates that there is either a major component failure (including a lack of, or disruption of, power) or there is a discrepancy between what the BIOS finds and what it "remembers" should be there.
This is why Essenbe recommends resetting or restoring the BIOS defaults. That process will clear up (refresh) any discrepancies, if this is the case.
Once the POST is complete the BIOS begins the remainder of the BOOT process: discovering installed components, drives, and devices of the system.
So it is very important for you to determine where in the start up process your system fails. And being an intermittent problem makes that more difficult.
If the system shuts down before you get that POST Beep, or before anything shows up on the monitor, then that indicates you have a problem in the power category. Could be your case switch, could be the connections on the motherboard (double check 'em!), could be the power supply itself.
To test the Power Switch on your case is pretty easy. Just pull the connector from the power switch on the motherboard (the PWR-SW header) and then use a small flat blade screwdriver to touch the 2 pins together momentarily. This should turn your system on. If the system displays the same problem when you switch it on this way then that would eliminate the case switch as a cause.
Let us know what you find.
