random reboots

My desktop PC suffers from random reboots during post.
What happens is that just when the blue desktop screen appears on screen, it will reboot itself to post again and Sometimes can occur in continous sucessions.
I have a abit kt7 raid motherboard, amd duron 800, 128sdram pc133, nvidia riva tnt2 model 64 video adapter, conexant hcf v90 56k speakerphone modem, 15gb maxtor diamondmax hard drive.
I have tried fdisk, format, reinstall software, diagnostic tools like dr watson, scanreg, scandisk, system file checker.
I would be greatful if someone could give me help on this matter.
thank you, varun