They've been annoying me for a while: some days the second PC would get decent net access and some days it would not, being either hideously slow or simply not work. I'd change something (like the proxy server or the cables) and it would work again (yay!) but the next day it would not (huh?)
So yesterday's trip was to see if the network hub (ex-conflux) was faulty: the hub was showing activity but the network diagnostics were saying there were numerous errors.
Link the PCs by the single cable and... nope, it's not the hub.
But ah ha - the network was going up and down in front of my eyes. It turns out that the socket on the network card (in the second, ex-conflux, PC) is loose. By wriggling the cable, you can make it work or not.
So this morning, I swap the network card for a 'network card plus hub on a PCI card' (ex-conflux) and it works. Even when you kick things and wriggle others. This frees up a power socket too, while keeping the potential for expansion from two to three PCs.
The only slight downside is that it's a 10M card/hub rather than 100M... but when you're sharing a 56k modem connection, this is not exactly the bottleneck!
So thank you again David and I'm going to recommend Ubuntu Linux again - the new release is due out next week - a nice combination of capabilities (everything the clients need plus tools for sorting out things like this, passworded so they can't mess things up too easily) at the right price: free.