Hi,
A few questions if this have not been resolved yet
1.Assuming these are all with the same HW spec did you try to disable cpuspeed or add -F to the test ?
2.have the driver work with 4k MTU. you can do in /etc/modprobe.d/modprobe.conf and insert "mlx4_core module parameter to set_4k_mtu=1" and then restart the driver
3.While the test is running you can use ibdiagnet and check if you have any re-transmissions or anything suspicious that might be a root cause