It sounds to me like you might have the CPU protection features enabled on your motherboard. Quite a few of the newer motherboards give you the option of monitoring the CPU fan speed and CPU temp, and if the temp gets too high, or the fan isn't spinning fast enough then it'll shut of after a couple of seconds. I tried enabling mine and got exactly what you are describing, but luckily realised after it shut down on me a couple of times as soon as I started. The fan probably hasn't had enough time to get up to speed before the mobo shuts it all off. Then every so often it manages to hit 2000 rpm or whatever the level is set to before the BIOS checks.
I simply cleared the CMOS (there should be a jumper on your board - in the bottom right part of the board in many). Just clear that and it'll reset everything in the BIOS so you'll have to make any changes to the BIOS again. You might be able to get away with holding down the insert key when starting it, but I reckon it may shut down before that works.
I hope that's your problem sorted! Let us know how you get on! Good luck!