WARNING*** I'm no IT Expert**WARNING

Ok, now thats outta the way......

I have an Asus PK5-E MB and had the same problems your describing, in fact sometimes it would not even POST.

MY problem was the Ram, the voltage in the default BIOS was set too low. I went through hours of random reboots or no POST at all... also fried 2 sticks in the process. What I did was go to my local computer big box store and buy a stick of cheap ram, installed it, booted to BIOS and set the correct voltage. Shut down, popped in my better RAM and everything worked perfectly. Returned the cheap RAM.

Hope this helps.... Good Luck!