Hi, I read this again and realized you are trying to get InfiniBand link up at 56Gb,,not Ethernet.
It sounds like a host-level issue (old mellanox driver, old adapter firmware, or perhaps a link quality issue causing the speed to degrade.
Check the host side port stats with "perfquery" to see if there are any discards or errors.
If there are, reseat both ends of the cable or swap it out if needed.