System freezes as BIOS loads

The system just freezes completely before the OS even starts to load.
Sometimes it starts up fine and when it gets into Windows everything works flawlessly, but getting in is tricky.

CPU: A10-5800k
MB: Asus F2A85M-LE

Tried resetting the BIOS and taking out alternate RAM sticks, but that doesn't seem to help.

This is my dad's machine, so this is all second hand info, but I just can't think what to suggest.
What would cause the BIOS to freeze but let the machine run perfectly fine in Windows?

That sounds like the BIOS is corrupt.
Have tried a reflash ?
http://www.asus.com/support/FAQ/1008280/

Ta, will give that a shot.

Are you able to load into the BIOS and look at the configuration.
If it keeps setting everything back to the default then it would suggest that your CMOS battery is dead/dying.