Debugging a simple net problem

[Ian]

I have a fresh install of 6.0-RELEASE on an old PII-450 machine I had lying
around.  It didn't have built-in Ethernet so I put in an old 3Com Etherlink
III ISA card into the remaining ISA slot.

I can get an address via DHCP from my router, but I can't ping the router.
I can ping 127.0.0.1, and the IP address the machine was assigned.  The
ifconfig and netstat -r look OK, I have addresses on my net going to ep0 and
a default route out via the router.

I'm puzzled that DHCP works but nothing else.  I configured the card using
sysinstall but I can manually configure the card with a valid address and
routing info, with the same result.  I'm not sure how to test further from
here.  Can someone tell me what I might try next?

I was considering replacing the network card, but since I can DHCP the card
does appear to work.

Thanks in advance,
Ian