Asus RT-AX88U Router will not maintain gigabit speed

Hello everyone.

I have an Asus RT-AX88U router updated with the latest manufacturer firmware and an Arris SB8200 modem.

I have the gibabit internet package and I know both devices are compatible.

I would like to know if there are some settings I am missing or if I need to do a factory reset as my gigabit speeds only last for a short period of time then are reduced to a fraction of their original speed.  Upon power cycling the router the speeds recover but, again only for a short period of time

Power cycling the router alone restores the speeds as well making me think it is not a modem issue.

On a fresh power cycling of both devices I show 6-700 Mbps downloads and 42 ish Mbps uploads.

However after several hours the download speed upon re testing is around 5 Mbps and 13 ish Mbps uploads.

These speeds were determined at the router via the internal speed testing app powered by Ookla.

QoS is off , Guest network is off, using a CAT6 cable

@EG I have seen some of your comments and was hoping you could possibly help me troubleshoot this.