RT-AC86U Internet "slowing" Over Days

[Doogeh]

Hi There,

I have an RT-AC86U running Merlin 386.5 firmware connected via Ethernet & Wifi to various devices. Over the course of a few days my Internet speeds slow from the usual 150Mbits/s (all times of day and night) down to 60-70Mbits/s on both Ethernet and Wifi. The "issue" is resolved trivially by restarting the router. I have performed ~10 speed tests, generally using https://www.thinkbroadband.com/speedtest, finding the issue, then rebooted the router to do another ~10 speed tests and get vastly different speeds.

The router is configured very much default, and I have made sure all QoS OFF, AI Protection OFF, no VPN, and with no USB devices or anything funky like that. I have done a reset to defaults a few times, but am otherwise a bit baffled with what is going on. The temperatures of the router seem to be stable with the CPU running at 73C constantly.

Can anyone provide some light on what might be the cause or a few things to try and resolve?

My internet is UK OpenReach FTTP with a single port Nokia ONT.

Thanks!