Regular internet connection drops with AC66U

Hello,

I've a rather strange issue with my RT-AC66U and the latest Merlin-Build (3.0.0.4.374.35_4): Every 15 Minutes the connection to the Cable-Modem (Technicolor 6700) and thus the internet drops completely and is automatically re-established after 30 - 60 seconds ("Timer" starts upon initialisation of the cable-modem). In the logs (cable-modem and AC66U) I do not see any suspicious entries.

What is interesting though is, that this behaviour stops and everything is back to normal when I remove any devices connected via cable on lan ports 1-4 and have only wireless devices working (and the cable modem connected).

The general set-up is:
1. Cable-modem in Router-Mode (Bridge disabled by Provider) and AC-66U configured as DMZ-Host.
2. AC-66U Connected via CAT5e Cable to the Modem and WAN configured as static IP.
3. (Unmanaged) Layer-2 Switch connected to AC-66U with various devices connected.

To check for cable issues, I directly connected my laptop with a CAT5e cable to the AC-66U (and removed everything else, except the connection to the cable modem), but the issue still persisted.

Any help and thoughts how to further address this issue would be greatly appreciated.