How about BIOS settings? Have you overclocked your computer? MacOS don't like it, >5% could cause instability, especcially during booting.
Try to reset your BIOS to defaults, if the problem is still there. You also can erase it completely (set the erase jumper on the mainboard, remove battery and pull the power plug for about 10 secs). Ist there a BIOS update available for your computer? What do you have 4 a board?

EPOX 9HEAI Rev 3 Mainboard

Mh... where can I find a BIOS update (Phoenix) for my board, and how can I install it?
I'll try with the BIOS-Resetting (only settings). It's not overclocked yet, but I planned to do so - I'll let it stay ;-)

-crawle

EDIT: 'scuse me ;-)

Which board manufacturer??? (Edit your post)

@naquada: I've edited it this morning ;-)