Raw Data,
I think you might have a RAM problem. The BIOS / POST test occurs before the OS gets involved at all.
I can only suggest trying a mate's RAM (ooer, missus!) to see if there is any improvement.
It is also possible that the main processor is not working correctly. I built a computer (this one, in fact) a few years back. Initially it had all sorts of problems and cost me a lot of time and expense until I discovered my new AMD was a bad one and replaced it.
Best of luck.