Glad my not-so glamorous 2 day troubleshooting experience with this helped you out :-)
When this happened, I had just moved my equipment from a shelf to a rack on wheels in my basement (due to construction of french drain).
My guess is that the pfsense/equipment was down for long enough time (full day before I rigged a consumer grade router to get temp Internet),
that the ISP decided to put me on a different subnet when I reconnected.
This drove me crazy as, with the move, I didn't introduce any new variables, but there was a physical change, none the less.
The symptoms would be that once I was able to turn things back on, I would get Internet connectivity, but then, once I would download a file and semi-saturate the link, the gateway monitor would check the old gateway from the original DHCP subnet that I was part of (I knew my IP could change at any time, but never imagined that they would also change your subnet).
I saw a bunch of WAN dropped packets in the managed switch that I use to connect everything, so I followed that route for an afternoon and changed cables, RJ-45 couplers, etc. I was almost to the point of suspecting AC interference due to the new cable routing!
Of course this was simply because the WAN would reject packets while the NAT states were being reset, but I had no idea of that yet.
It was not until the weekend when I was able to do more testing and debugging, that I realized what was happening.
This never came up when I did the upgrade to 2.3, as my WAN gateway had not changed, so I just could not imagine what could have changed
from the equipment being on a static wooden shelf, to being on a mobile wire shelf :-).
Good lesson, just hope I don't get more of these crazy ones!
@ironashram:
Gateway monitoring indeed was my problem, we have nexus 9000 in our new setup and they bring this fantastic feauture thak makes gateway respond to ping only sometimes :(
Thanks pppfsense for pointing me in the right direction.