It's the cables, stupid
Improving my datacenter in the basement, I installed a proper Firewall instead of the dinky 4-port NAT router that connected our house to the wild intranet. I used an Astaro Security Linux on an old PC, a setup that has served me well in the Office network. To make everything look really good, I used red and green network cables that I had salvaged from somewhere.
Installing the firewall was straightforward, I copied the settings I use at the office and just changed from PPPoE (ADSL) to Ethernet Interface (Cablecom) and was set, more or less.
Soon after we had internet and I continued to work. But - something didn’t seem right. I had intermediate problems, time-outs, drops of connections. MTU size, I thought, having seen similar symptoms a couple of years ago. So I switched from 1500 to 1492 to 1400 back to 1500. No joy, as Jerry Pournelle would have said.
I said to myself (and to anybody who would listen): it feels like a loose cable.
And finally my friend Paul said: Have you tried switching the cables? Then it dawned on me: Those fancy colored cables… I replaced them, rebooted Firewall and Cablemodem for good measure, and lo and behold - all back to normal.
The cables are in the trash. Always check the simple things first.