Heavy traffic causes ethernet link to go down

I have a FreeBSD (10) host connected to a Linux (Debian 7) box directly with a Cat 6 cable.

Both hosts have Intel X-540 10GBe NICs (ixbge).

Whenever there is heavy traffic (high PPS) from the FreeBSD side, the Link state between the adapters go down. Setting FC (Autoneg, RX/TX, all) off seems to not have any effect on this.

Neither OS kernels log anything useful, only message is about <interface>: NIC Link is Down After a half a second or so, the links goes back up.

1. What might be the cause? or,
2. how to troubleshoot this?

EDIT:

Tested with packet generator: It seems that it is exactly 8188 packets that get through and 8189th causes the link go down, regardless of packet size

EDIT 2: This seems to be the transmitting end (FreeBSD) that is to blame, connecting the port in question to another port on the same host yields the exact same results.