Sort of sounds like my issue. Under
LAN/Switch Control, do you have Spaning Tree Protocol Enabled?
I've had to disable mine in what seems to have solved my funky Speedtest results and like you testing at the modem and the modem stats showed everything was fine...
Won't know for sure until I enable it again and break it but so far so good, but ~930/~930 for almost two days straight and haven't seen it drop. When it fails my download is cut by 1/2, and my upload by 1/4 at the router if it finishes. At the desktop/laptop (wired) I get the full speed with minor packet loss. A reboot fixes it as well, until a topology change occurs and then it's back to funky results...
View attachment 49428
Why the topology change? I'll be digging into that tonight/tomorrow...