At the time of my last posting, I had been hitting this really weird kernel panic every time I'd connect the ethernet cable to the WAN interface. Sometimes it would happen immediately, sometimes within several seconds, sometimes not for 20 seconds. But it would happen, always upon WAN connection.

So, I tried:

1. Updating the BIOS
2. Disabling ACPI at pfSense boot - No option to do this via BIOS. Even for such an old heap, the system board's firmware is pretty garbage. It has Cool'n'Quiet. Always disable C'n'Q.
3. Disabling all unnecessary devices via BIOS (including onboard NIC and all floppy support)
4. Adding more memory - this finally worked, perhaps because of the BIOS update. But I think I also had a bad stick in my junk pile. Now at 2 meager GB. I'm pretty certain this thing can be maxed out around 12GB (users have reported 3x4GB sticks work, but adding a fourth doesn't)
5. Altering settings permanently via /boot/loader.conf.local:
   1. hint.acpi.0.disabled="1" (see #2 above)
   2. reviewed USB boot issues here - nothing really seemed to match.
   3. Tuning the network
      1. kern.ipc.nmbclusters="1000000" (this sounded exactly like what was happening)
      2. hw.em.fc_setting=0 (disabled flow control - for PT/em-based card)
6. Changing the IP address and releasing the DHCP reservation (was connecting WAN via a subnetwork through my existing router)

The new NIC arrived today, so I thought what the hell. Sure as shit, popped it in and it works straight out of the new old box. So far.

I still need to configure the LAN but I have web admin configuration access once again and the thing seems to be pretty stable.