@TASan 

You should replace Ethernet cable that connects Main Deco to the router. When you restart Main Deco, it negotiates gigabit speed with router over that cable. Later, due to issues with cable (transmission errors, packet loss) router and Main Deco automatically downgrade speed to 100Mbps. When you restart Main Deco, that cycle repeats.

The most common reason for router and Main Deco to downgrate connection speed between them would be Ethernet cable issues.

Thanks @Alexandre, you pointed me in the right direction. Cable issue for sure, but finding WHICH cable will take some work.

My setup is more complex:
 

Modem -> Wall Box -> [Attic Cable] -> Wall Box -> Router -> [Cable] -> TP-Link Switch -> Deco + other devices

When the problem occurs, BOTH my Deco AND devices behind the switch drop to 100 Mbit simultaneously. This means the bad cable is upstream of the switch, not the Deco's

own cable.

How I'm diagnosing it:

1. Speedtest monitoring - Cron job runs speedtest every 30 minutes, emails me when speed drops below 250 Mbps
2. TP-Link TL-SG108E switch diagnostics - When alert fires, I check:
   - Port Statistics (Monitoring -> Port Statistics) for RxBadPkt errors
   - Link status on uplink port (100Full vs 1000Full)
3. The key test:
   - If switch uplink shows 100Full = cable between router and switch is bad
   - If switch uplink shows 1000Full but speed is still slow = problem is upstream (my attic cable)
4. Confirmation test: Plug laptop directly into router and run speedtest. If laptop gets 500 Mbit but switch devices get 100 Mbit, the router-to-switch cable is guilty.

The tricky part is it's intermittent. Cables test fine when working, then randomly degrade. I have to catch it in the act.

Will update when I find the culprit. Hoping the monitoring catches it soon.