If you notice in the BIOS of the A7A266, 12.0 is the maximum you can set the multiplier to yourself, with 12.5 being reserved for auto. If you want to use a 2400+, you need to set it to "12.5x (or higher)", or try auto instead of manual. I believe I had problems when I set a 2000+ in the same board to manual.
I forgot about that little quirk.