Kernel configuring

I'm trying to reconfigure a "backup" HD system to replace a failed HD. It's
pretty close but the network connectivity isn't working yet. I see the
configuration of eth0 fail during bootup and boot.log contains the following
relevant entries:
Nov 23 04:58:45 Halkin sysctl: net.ipv4.ip_forward = 0
Nov 23 04:58:45 Halkin sysctl: net.ipv4.conf.default.rp_filter = 1
Nov 23 04:58:45 Halkin sysctl: kernel.sysrq = 0
Nov 23 04:58:45 Halkin network: Setting network parameters: succeeded
Nov 23 04:58:46 Halkin network: Bringing up interface lo: succeeded
Nov 23 04:58:46 Halkin ifup: SIOCSIFFLAGS: Device or resource busy
Nov 23 04:58:46 Halkin ifup: Failed to bring up eth0.
Nov 23 04:58:46 Halkin network: Bringing up interface eth0: failed

Since the configuration of lo succeeds, it seems to me that the basic
network stuff is enabled. I'm unsure but I think that from the "ifup"
messages, it's trying to tell me that the setup of the ethernet card failed.
Is this correct or does it indicate a different problem?

If the error indicates a problem with the NIC, I'm confused because I'm
pretty sure that I've built into the kernel (as "included", not as "module")
the correct NIC components. The card says "3COM Etherlink III 3C509B" and
in the kernel configuration file I find
CONFIG_NET_VENDOR_3COM=y
CONFIG_EL1=y
CONFIG_EL2=y
CONFIG_ELPLUS=y
CONFIG_EL3=y
indicating, I think, that the 3Com card support should be there.

Since a different kernel in the same box has a working network every time
it's booted, I'm positive that it's not a hardware issue like interrupts or
ports.

Is there a kernel networking option that I've likely missed setting?

All help, suggestions, hints, etc, graciously accepted.

TIA
Norm