@mkent:
The pfsense reverted back to a DHCP IP address last night about 3am of it's own accord.
The behaviour you describe is seriously unusual. :o In your place my assumption would be that hardware is failing or that pfSense was corrupted during the original installation. If I was in your position and had a firewall behaving that way I would check the drive for problems, format the drive and reload it from scratch using install media that I'd verified the checksum on beforehand.
If the problem persists after a full reinstall, I'd suspect either a driver incompatibility or a hardware issue which I would then start looking for in the runtime and boot logs for inconsistencies or errors.
Here are the possible hardware causes that come to mind in troubleshooting order based on my personal experiences:
a. Cabling and/or switch (use a different port and cable)
b. LAN NIC (To verify, try swapping the LAN and WAN assignments using the console menu item #1 and see if the WAN then exhibits the same issue.)
c. Hard disk (check the logs for errors, use Diagnostics: SMART Status: Perform Self-tests, badblocks*)
d. RAM (replacing is the often the surest way to confirm)
e. BIOS bug/corruption (reinstall/upgrade bios)
f. Motherboard
badblocks can erase all your data depending on options but is great at finding drive media problems
pkg_add -r e2fsprogs
http://www.freebsd.org/cgi/man.cgi?query=badblocks&sektion=8&apropos=0&manpath=SuSE+Linux%2fi386+7.3