Your Sempron 2500+ runs at 1743 MHz: it has a multiplier of 10.5 and needs a bus speed of 166 MHz (which means 333 MHz because it's DDR).
Given the multiplier of 10.5, it's pretty obvious you have a problem with the speed of the front-side bus.
According to Asus, you need bios 1003 or higher to support the Sempron 2500+.
What I would do is first check the bios revision and if it's okay, check the bus really is set to 166 ("cpu external frequency" is unclear). Asus bios downloads and manuals here: click here