500Mbps dropping

G'day

I recently upgraded to 500Mbps and all was fine.

Noticing some slowdowns, I ran SpeedTest and it reported wired connection speeds of around 140Mbps.

My Virgin router is in modem mode and I'm using a ASUS router.

Ethernet cable is fine. Speed test done multiple times. Rebooted routers. Checked relevant settings on router.

Any suggestions?