If you are using gmail's outgoing mail server, smtp.gmail.com, I don't see where Plusnet comes into it. As you say, it used to work, so the only things that have apparently changed are a new mail client on a new PC. It would be worth trying a different client, e.g. Thunderbird, to see if you get the same problem, but intermittent problems are always the worst.
plusnet Extra 80/20 trial