It happened again this morning.
It looks like the problem was one of our lines - despite the modem saying it was connected at 3700 kb/s, throughput to the ISP's speed tester was more like 500kb/s. The ISP has done something, and it is better.
The situation is complicated by the fact that my access to the system is through a wireless network to where the router is, a mile or two away, and the router load balancing means the problem comes and goes, depending on which line a given connection gets.
However, to help diagnose, I put a temporary firewall rule to direct all traffic from my IP through a particular modem, and, by editing the rule, was able to pin down which modem it was. I have kept the rule (disabled) for future use if it happens again.
I am still mystified as to what I might be doing wrong with the traffic limiting wizard, that it would halve the bandwidth, so any suggestions gratefully received.