Barton XP 2500+ Week 35 Appears to be Locked--on a 266 MHz mobo

I just picked up a Barton XP 2500+ W 35 from mediantech. I am running it
on an Epox 8K5A Mobo, which only supports a 266 FSB. On friday I will swap
it into an Epox nForce2 board (no model number given) with 400 MHz DDR.

I can adjust the FSB on the 8K5A to 166 MHz, and I can alter the multiplier
and have the system boot. The best I can do is 166x11 on this board.
Although the bios shows a 13x multiplier setting, any setting larger fails
to post and setting lower has no effect on the cpu frequency.

Is the problem the Mobo or the CPU?

I swapped out an XP 2000+ chip, and with the larger cache and faster bus
speed, the system shows a 10% improvement in calculation speed.

I was hoping for a 25% improvement before I started overclocking. The
numbering system of these chips are a bit optimistic, but I will see what
happens, when I put it into a decent Mobo.

Pete