I looked at my AC3200 logs from last night where the router went offline twice for a few hours at a time. The best I could ascertain was I think I was getting some STP related events or similar: (this is just an example)
Mar 6 15:09:43 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:46 kernel: net_ratelimit: 2 callbacks suppressed
Mar 6 15:09:46 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:46 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:46 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:46 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:53 kernel: br0: received packet on vlan1 with own address as source address
Mar 6 15:09:53 kernel: br0: port 1(vlan1) received tcn bpdu
Mar 6 15:09:53 kernel: br0: topology change detected, propagating
Mar 6 15:25:41 kernel: br0: port 1(vlan1) received tcn bpdu
Mar 6 15:25:41 kernel: br0: topology change detected, propagating
Mar 6 15:25:53 kernel: br0: port 1(vlan1) received tcn bpdu
Mar 6 15:25:53 kernel: br0: topology change detected, propagating
To my knowledge I'm not explicitly using distinct VLANs. I have about 25 devices with 3 ethernet switches in different rooms. Shouldn't be any loops there, though I also have 4 power line adapters to connect rooms (AV2000). I have a Virgin media SH3, and tivo v6 box, Sonos*5, google home*5, Chromecast*2, xbox, several laptops, pcs, smart lighting , raspberry pi.. all kinds of things!
The times of the failures (and in fact the subsequent self-fix ... there wasn't a reboot here) seem to align with these kind of events, though overall there were a LOT in the log
Perhaps a punt, but decided to switch STP off for now. Not seen any of these kind of events thus far. Let's just hope there isn't any looping possibility!