Thanks to everyone for the suggestions, it eventually turned out this was just cable internet provider doing what it does. Customer complains, they turn the speed up for a few days, and it shows a bit over 200Mbps , then over the course of another 2 weeks or so it drops back to under 100Mbps. Customer makes another call, tech shows up, they turn the speed up again for a while and say everything's OK, rinse & repeat. I went ahead and did the upgrade to pfSense 2.6.0-Release to stay current, but both pfSense and the hardware it's on are working perfectly, everything else was typical ISP shenanigans.