Hello,
I've had a Vigor 2850n at home for about 5.5 years and use a site-to-site VPN with the same router at work.
It connects through Fibre Broadband with Zen Internet.

On a few occasions over the last couple of weeks, the router goes into a state whereby:
1) internet "appears" to drop out (pinging 8.8.4.4 shows a mixed bag of "Time outs" and replies with high latencies).
2) pinging the server itself through Gigabit LAN also shows hugely long lag times of 1000-3000ms (usually <1ms)
3) web interface is not responding/taking ages to display the login page
4) even tried SSH using Putty.exe and that takes a minute or more to respond - or connection is terminated.

A reboot doesn't fix the problem so I end up switching it off and running my Zen-supplied FritzBox for a day or so (which runs fine with solid connectivity).
When I come back to the router, it's fine for a bit (a day or two, or several hours) and then it starts playing up again.

I looked for a firmware update following the CRFS/VPN attacks (https://www.draytek.co.uk/support/security-advisories/kb-advisory-csrf-and-dns-dhcp-web-attacks) but there isn't a firmware update and the product is considered "legacy". Other than that, it is running the latest firmware.

I tried saving the backup, doing a factory reset and restoring backup but that hasn't stopped it happening again.

Can anyone suggest a cause? I'm sure it's not Broadband related and I'm bypassing WiFi when trying to access it and the internet.

Thanks, Chris