8247 ether_fcc underruns, but only running kernel

I have an 8247 which when running u-boot works fine on ethernet, both 10mb
and 100mb.  It can dhcp and load a kernel.

But when I boot the kernel (2.4 or 2.6) the fcc transmits get an underrun
everytime.  Packets are received fine.  This is on FCC1.

(I'd suspect the tx clock, but ethernet is ok with u-boot.  I double checked
the clock configs under linux.)

Does this ring a bell with anyone?  I started out with rstconf#
grounded, (i.e. default speeds), but i also tried both the cpu and cpm
at 166mhz.  Both acted the same.

I've no doubt got something misconfigured, but what...  I've tried different
BCR settings.

-brad