Well, not an academic issue as its the primary firewall for my home office and it seems silly to limit bandwidth below a certain rate when the equipment (not to mention paying for it) should be able to handle it.
That being said, within 4hrs of the initial post, the firewall killed again, without artificially saturating with a speedtest. Additional testing with hard setting the rate to 1000 didn't seem to have an effect. Something over 100mb/s and obviously less than 1000mb/s kills the thing.
At this point, reviewing my old notes and logs from a while back… and actually forum posts here. The original WAN port was bad, so with the current symptoms and it not seeming to be a pfsense issue, pretty sure the old IBM appliance is just giving up the ghost.
As such, think it is time to replace... so just going to mark this as solved so folks don't attempt to go down a rabbit hole.