NF7 - power problem

[Ceefer]

I wonder if any of you have seen a problem like this?

I've had an NF7-S motherboard lying around for a couple of years doing nothing so I decided to sell it on Ebay. Being an honest soul I thought it a good idea to test it first, so I cobbled together a basic system with an Athlon XP2500+ (stock cooler), a stick of DDR400 memory, a Geforce3 gfx card, an old CD drive, a 40 gb HDD and a 450w Enermax PSU. Pressed the power on button and nothing ...no lights, no fans. Rats, I thought, must've connected up the front panel incorrectly. I was just thumbing through the instruction manual looking for the diagram showing the front panel connections when the computer started up! It must have been a couple of minutes at least since I pressed the power button. So, I hit DEL to get into BIOS, set everything to where it should be (ie no overclocking or anything), popped a Windows XP CD in the drive and was just about to install Windows when the computer powered down. So, I disconnected everything and put it all back together again, this time with a 400w Macron PSU. Pressed the power button and again nothing happened at first but the computer started up after a minute or so. This time I managed to almost get Windows installed before the machine shut down. I've repeated this a few times and the motherboard never powers up straight away and just won't stay powered up for any length of time. I've changed out all the components except the case. I've cleared CMOS as well.

What could cause this? Its not the PSU because I changed it. Its not overheating, temps are normal. Could it be a flaky capacitor or something?

[BUFF]

tried a new CMOS battery?

[Max Tractor]

IIRC 1 stick of ram equals slot 3, what slot are you using?

[Ceefer]

Hmm....I'll try a new CMOS battery.

I think I put the stick of ram in slot 1. I can't see why it should delay the power-up if it was in the wrong slot. However, this does make me realise that I should rtfm!