I found a copy of the manual on-line, but couldn't get it to download.
You downloaded the latest BIOS, so that rules out the semperon/compatability problem that I seen on some other websites.
I know I sound like a broken record, but I'd check the power supply.
A friend had a similar problem. He couldn't get his pc to work if he changed the FSB to 333mhz, but had no problem running it at 266mhz. Long story short, turns out the power supply wasn't powerful enough. By fiddling with some power settings in the BIOS we managed to get it to work until his new power supply showed up. Now everything works fine.
You could check the memory, double check the temp, double check the jumpers and fiddle with the BIOS settings, but my money is on the powersupply.
How much "stuff" do you have in the pc? How powerful is the power supply?