Do you happen to use AV software or a transparent proxy that scans https traffic? That's usually done using a local CA certificate so that the AV software or proxy can MitM the connection. I've seen intermittent errors caused by buggy implementations of that. More specifically, your Firefox error code has been connected to some Kasperksy products (but not necessarily just limited to that).
From what I can tell, your server configuration looks fine, so I'd look into client-side issues first.