Agreed about Zen, we experience random dropouts at night sometimes for hours, I don't know if they are doing unannounced infrastructure changes or what it is.
I've observed it from the other side (failing to connect to a Zen customer for more than an hour), and additionally the zen.co.uk website refused to serve me either directly or via Tor when I tried to check its status and/or a looking glass service (though not recalling finding those anyway, when I've finally connected via a private server).