Turns out the power supply fan in my firewall machine has seized. I guess things got too hot inside and shut down. So, cover off, vacuum dust out, upstairs a/c on, ceiling fan on, and after letting it cool down for a few minutes with the power off, it seems to be stable now.
My firewall machine is a Dell Pentium 90 that is about ten years old. I wonder if I can still get a replacement power supply for that machine.