DFI AK75-EC motherboard

One of my machines is running a DFI AK75-EC mobo, 512 meg ram, Athlon XP1900 cpu, Ubuntu 9.04. This board has a switch for the cpu multiplier and the FSB can be set in the BIOS. The XP1900 was running at 1.6MHz at 12.5 on the multiplier and 133 FSB. I swapped in a Athlon XP-M2400 and with no other changes it ran at 1.7 MHz. V-core is set at default. The 12.5 multi is the max setting on the board. I changed it to run at the default setting for the 2400 cpu and it wouldn't boot. Cleared the CMOS and set the multi back to 12.5 rebooted, now it runs at 800MHz no matter what I do.
Any ideas ?
I know this is some pretty old hardware, but, fun to play with, none the less.

 

Well, I think 1.6GHz and 1.7GHz is more like it.
That's what happens when you get in a hurry and
don't proofread before you post !!