Here's what's more strange — installing a PCI-E network adapter caused the defunct adapter to start working. I've seen this on the other two machines as well. I can install a PCI-E adapter, or a wireless adapter, and this somehow causes the onboard NIC to work again.
I purchased and installed this NIC: www.amazon.com/.../ref=pe_175190_21431760_M3T1_ST1_dp_1