@bortzmeyer
Right now, no, but it has come and gone before, I think it will come back. I'll post when I can do it again and try to bring in more data.
So far, the issue has been narrowed down to just one resolver, 91.151.130.117 / maradona.silknet.com.
It does not always show this behavior, and it is not always the only resolver provided by the ISP, but it has happened at least several times in the last 4 days and lasted for several hours