New Chip and Motherboard Problem

Just bought an Asus TUV4X mobo’ and a Pentium III 1.2 GHz processor as an upgrade.
Put them in my existing system, 40GB HD, Geforce2 MX400 video, 768 MB SDRAM, and fired ‘er up.
No problems, temperature is just right.
I go ahead and install XP Pro and driver updates, etc. NO problems.
Then I decide to flash the BIOS to latest version.
Immediately after the flash, and loading setup defaults, I exit, and upon restarting, it won’t post or give me video.
Shutting down, turning back on let me get back to setup but now the chip will only run at 600MHZ.
Went back to old BIOS ver. did nothing to help.
HELP.