@dee.jay - this is weird...
So I have two sites, both with pfSense and connected via 1GbE. They've often run file transfers across without ever an issue before. Just ran iperf between them. I also connected an iperf client to the affected gateway to further load it. I also started a ping out to 8.8.8.8 and to the gateway to verify LAN and WAN connectivity.
Interestingly I see dropped packets on both LAN and WAN:
LAN
64 bytes from 10.1.0.1: icmp_seq=5 ttl=64 time=0.646 ms
64 bytes from 10.1.0.1: icmp_seq=6 ttl=64 time=0.613 ms
64 bytes from 10.1.0.1: icmp_seq=7 ttl=64 time=1.183 ms
Request timeout for icmp_seq 8
64 bytes from 10.1.0.1: icmp_seq=8 ttl=64 time=1488.186 ms
64 bytes from 10.1.0.1: icmp_seq=9 ttl=64 time=487.726 ms
64 bytes from 10.1.0.1: icmp_seq=10 ttl=64 time=0.674 ms
WAN
64 bytes from 8.8.8.8: icmp_seq=41 ttl=54 time=4.419 ms
64 bytes from 8.8.8.8: icmp_seq=42 ttl=54 time=3.936 ms
64 bytes from 8.8.8.8: icmp_seq=43 ttl=54 time=6.613 ms
Request timeout for icmp_seq 47
Request timeout for icmp_seq 48
64 bytes from 8.8.8.8: icmp_seq=44 ttl=54 time=5207.083 ms
64 bytes from 8.8.8.8: icmp_seq=45 ttl=54 time=4207.624 ms
64 bytes from 8.8.8.8: icmp_seq=46 ttl=54 time=3207.687 ms
To reiterate I'm not sending any traffic on WAN at all.
After running the iperf test for a few minutes both LAN and WAN have gone down on the pfSense box.
Guess the issue is definitely a pfSense one...