Just a thought as you mentioned that the problem is when you edit the BIOS. Try making sure that your two BIOS chips hold the same version of the BIOS file.
To check, go into the BIOS and on the MAIN page press F9 for system info. You should see both BIOS versions listed, if they do not match please update the backup BIOS to match the current.
To do that, reboot and where you would normally press DEL to enter the BIOS, instead press Alt + F12, this will flash the Backup BIOS with the MAIN BIOS contents. Do not worry when you see a recovery comment, that is normal and means it is flashing the backup BIOS.