igc problems with heavy traffic

[Jim King]

I'm trying to use a Qnap dual port 2.5Gb NIC with FreeBSD 13.1. FreeBSD 
recognizes the NIC and I'm able to connect to my LAN at 1Gb.  However, 
trying to push heavy traffic through the NIC results in a temporary loss 
of connectivity:  traffic stops and the message "igc0: link state 
changed to DOWN" appears in the system log.  About 10 seconds later the 
message "igc0: link state changed to UP" is logged, followed by some 
messages from dhclient indicating that it has acquired a lease.  At that 
point network connectivity works normally, until I try to push a lot of 
traffic again.  I've been able to trigger this every time using iperf3 
from another PC on my LAN, and by using Ookla's speedtest client (my 
Internet service is good for about 900 Mbps).

With a recent version of Ubuntu I'm able to do both of these tests with 
no problem.  As far as I can tell the NIC itself is OK.

Is there anything I can do to make this NIC work with FreeBSD?  I was 
hoping to use this for an Opnsense router, but at this point it seems to 
me that the igc driver just isn't robust.