I've just added a new motherboard to my pc with with an 2600xp amd chip. The chip was previously in my old computer which had another board that didn't support this part
The chip is in the new motherboard but not running as fast as it should clocking at around 1.26ghz.
What could be causing this?

Thanks for the help

There will be a switch or jumper that controls the front side bus speed. That's the only reason I can think of unless it's actually running at the correct speed but the BIOS is simply recognising it as a slower chip. Check your motherboard manual for CPU and bus speed set-up, if not update your BIOS version.

Hope this helps.

There are 2 versions of the 2600xp. 266 n 333.

The multiplier in BIOS should be set to 12.5 for the 266 variant and 16 for the 333 CPU.

CPU true speed FSB Multiplier
2600+ 2133 133 16.0
2600+ 2083 166 12.5

DDR RAM is actually (often) quoted at half it's speed as the RAM uses leading and falling edges of the FSB wave. Anyhow, using my best guess I think your system has being setup incorrectly. You'll probably find these are your settings:

CPU Mhz FSB Multiplier
2600 1260 100 12.5