What is preventing my computer from switching on

Hi, I have a computer, a Shuttle SN85G4 with a socket 754 Sempron Processor, 256mb Ram, an 80Gb SATA hard disk. When I go to switch on my computer the blue power light flashes, the fans spin for a fraction of a second and then it switches off, further depressions of the power button produce no affect whatsoever. If I pull the power lead out and plug it in again the same happens, but after a few attempts it will boot up though the number of attempts varies to produce a result. Once it has booted up it runs fine.
I suspected the power supply and swapped it out, I thought it seemed better for a day or so, but now it is the same so I have replaced the original power supply. I have tried swapping the ram to no effect. I have removed the motherboard and cleaned and reinstalled it. I notice the bios reports the sempron processor as an unidentified Clawhammer type processor. Could a CPU cause these types of problems? What else could cause this?
textusdominusAsked:
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CallandorConnect With a Mentor Commented:
I agree with Frosty555 - you have a power problem, typically caused by bad capacitors in the power supply or motherboard.  The key is the fact that you are able to power up after removing the plug and it keeps running once it starts.  You swapped the power supply, so that leaves the motherboard, and that's what you have to replace.  The motherboard obviously has other problems, such as recognizing your cpu, but you need to replace the motherboard.  See www.badcaps.net for examples of bad capacitors (bad caps aren't always visible on the outside, though).
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Frosty555Connect With a Mentor Commented:
I've experienced this on two computers before. Eventually after testing everything I could I decided it was the MB that was the problem and replaced the machine.

It may or may not be the case for you, but I would suspect the motherboard. Incorrect detection of the cpu I think also points towards a motherboard error. Perhaps try resetting the bios on the motherboard (via pin jumpering, check the manual).
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textusdominusAuthor Commented:
I have never flashed the bios and the processor is newer than the board, so maybe a bios update would resolve that. Could a incorrectly detected CPU cause these kinds of issues? Doesnt the processor send some kind of power good signal to the Power supply?
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Frosty555Connect With a Mentor Commented:
Updating the bios to the latest revision is probably a good idea. I'll find you the website. There is a windows based tool that does it for you.

The jumper thing is to reset the SETTINGS. It's not a reflashing at all, you just place a jumper on the motherboard in the right place and it resets all the cmos / bios settings back to factory defaults.

Here is the website with info on your MB - bios updates and the windows tool etc.:
http://eu.shuttle.com/archive/en/sn85g4_faq.htm

And the manual is here. Apparantly the MB is model # "FN85"
http://eu.shuttle.com/download/Archive/Manuals/en/fn85/fn85en.zip

On page 26 of the manual (based on the numbers at the bottom of the page), is instructions on resetting the cmos settings by using jumper JP11 on the motherboard. Follow those instructions. A summary of the instructions:

1. Turn off system
2. Remove ATX powre cable (remove power cable from motherboard)
3. Remove jumper cap from JP11 (pins 1-2)
4. Place jumper cap on JP11 (pins 2-3)
5. Wait a few seconds
6. Remove jumper cap and replace where you found it (JP11 pins 1-2)
7. Reattach ATX power cable and power on the system

But look at the manual I gave you. It has diagrams on where you can find this jumper.
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