I was able to solve it about one week ago. Detail below, maybe it can helps some other users.
I previously called two times the telephone support, explaining the problem (high packet loss, with correlation between [packet loss TO modem] and [packet loss TO internet servers]. Suggesting that packet lost was already happening into the modem.)
They were not able to fix it (well, they proposed some modem reset, move some cables,etc..). and eventually proposed a technician in order to verify the line. I’ve explained that probably line was not related. Packet loss only happened in burst (with 10-50% of loss), so it was not easy to reproduce after each trial.
I continued to investigate.
The problem was an update of a program, combined with some performance problem of modem. That program started to send lot of DNS requests (well, for sure <1Mbit/s, as this is the overall consumage of the device. And my connection is 300Mb/s). But I suppose that problem was related to lot of opened/closed UDP connection (which caused some problem to firewall of modem, I suppose).
But now I’m curious: your technical support is able to inspect error ratio per port of modem (and see devices attached, useful in order to help customers). But is not able to detect high CPU/ram usage of modem, or number of opened connection? (which would be for sure triggered by my problem).
By the way, now packet loss is very small (but not zero..sometime some peak of 1%, so sometime I lost some vpn/ssh)..but I think this is a common problem for coaxial connection.
