I would suggest doing the latest revision BIOS updates. If i remember correctly when i had this board in my gaming rig 1GB modules were not available.
It is possibly a problem addressing the 1GB module you can download the BIOS update from this site
http://support.asus.com/do wnload/dow nload_item .aspx? prod uct=1&mode l=A8N-SLI
I hope this fixes your problem.