Similar thing was also happening to me on one of my servers also with a dual port Mellanox, every few boots a new eth interface would be detected for port2, i.e., NIC that was eth4 became eth5, then eth6 and so forth, each time this happened I needed to reconfigure the last NIC detected as the previous one stopped working, my "solution" was to swap that NIC to a server with a different motherboard and it never happened again, firmware update might also help.