strange, cuz it works okay once in windows.
floppies are known to go bad, at least certain sectors.
many ppl suggest loading set-up defaults both before & after flashing.
yanking the battery for 30 mins should get you back to default-land.
try the guys at asusboards.com.
http://www.asusboards.com/vbulletin/that's where i go to get help with my asus boards. nice group of guys there. very helpful.
also, realize that there are several different bios versions, cuz there are several different versions of the cusl2: -c, -m, etc. Also, each board has a separate version for those with onboard sound, and those without. Make sure you're using the correct version. I used 1009 without any problems. Altho, I think I first had to reboot a few times.
I am now using the Asus P4T-E mobo (bios v1005) with a P4-1.6A Northwood (o/c to 2138MHz).