DHCP fails under 2.6 kernel but works with 2.4

I bought a new motherboard and CPU (Sempron 2800 + Mercury/Via 
KM800M)
after my old motherboard (Athlon XP2000 + Epox/Via KT266) stopped 
functioning and re-installed Sarge.
Now I find that the 2.4 kernel works perfectly but
DHCP fails at the DHCPDISCOVER stage after a few tries
(it realises that the link is up and proceeds to search for the 
server)
when I boot 2.6.
Please help.