I think interference from another driver is more likely. The SRD does load some drivers at boot time, and there have been issues with different manufacturers NICs that use similar networking chipsets.. An already loaded driver may be interfering with your attempts to load the proper driver. The earliest version of the SRD that I have is version 8.5.1 but yours may be an earlier version.
Without having your version of the SRD to test with, it would be hard to test at this end. The drivers should encompass any variations in the NIC's firmware that occur ed during the production run of the Optiplex 760's mainboard. Perhaps you should try updating the firmware for the mainboard and see if that makes a difference.
Beyond that, I think Chris Riley's suggestion about testing with the version 8.5.5 SRD is your best hope. Please keep in mind that you would not need to install the full version of the product in order to test the newer SRD. The newer SRD will restore images made with older versions of this product. When downloading the trial version, please be sure to download all the ISO images presented. I believe there are three. One of those ISO's is the SRD. If the newer SRD does work with your equipment, then you have a stronger case for upgrading. You've indicated that several weeks have already been spent trying to get your current version to work. Providing the newer version works, the current number of hours spent on this project would have to be close to the cost up upgrading the product.