Thank you for your generous offer, but after days of incessant searching I have finally found what I believe to be the BIOS update I needed! For the benefit of those who read this thread in the future, I will explain how I found it:
By searching for "Intel Lexington 3 BIOS update" on a search engine, I found an old, dead link to Gateway's site which at one time had the BIOS update:
http://support.gateway.com/support/drivers/search.asp?param=bios&st=kw (dead link)
Upon viewing the page on Archive.org, I found a download link for a Lexington 3 BIOS update on an old version of their page, from June 6th, 2008:
That Archive.org link lists a Lexington 3 BIOS version MV85010A.15A.0086.P15, which is newer than my current version, MV85010A.15A.0041.P05, and is also the newest version I found. Other web searches have lead me to believe that this is the last BIOS update released for this motherboard. However, Archive.org didn't archive the file that the above download link points to.
Now armed with the version number I needed, I did another web search for "MV85010A.15A.0086.P15". All it returned were spam sites and the like. So to try attacking it at another angle, I instead searched for the name of the file (9521644.exe) from the above Archive.org link, as opposed to the BIOS version number. Simply typing 9521644.exe into a search engine let me to this site:
http://www.driverguide.com/driver/detail.php?driverid=883489
On that page I downloaded the heretofore elusive "9521644.exe". I manually extracted the .exe with an archive manager to verify that this was the file I was looking for. I found a readme.txt. Here is a couple excerpts from the readme.txt which verifies that this is the BIOS I was looking for:
9521644.exe
readme.txt
File Description: Lexington, Lexington 2, and Lexington 3 Motherboard BIOS Update
Revision: MV85010A.15A.0086.P15
Operating System: No specific Operating System
Driver Part Number: 9521644
Date: 12/02/2002
9521644.exe
readme.txt
Overview
This is the P15 BIOS update for the Intel motherboards with a BIOS identifier of MV85010A. These motherboards are the Lexington, Lexington 2 and Lexington 3 motherboards.
While the release date shown in the readme.txt and the Archive.org link don't match, everything else including the version number are identical. I am going to give it a shot this weekend, and I shall report back with the results.
Thanks so much for your help!