Have to reboot every 2-3 weeks to get back to internet tier I pay for

I notice my Internet is running slow. I perform a speed test directly Ethernet connected to the Xfinity Internet tower and get results in the 20-30Mbps range. I get similar results over WiFi. I then power cycle the Xfinity Internet tower and then run another speed test and get the 600Mbps that I pay for. Why does this happen every 2-3 weeks ?