what device are you testing on? 

Have you tried other devices? 

And/or test servers? sometimes speedtest doesn’t always pick the fastest server. So its worth trying several servers when you’re getting over that 500mbps service

You can also try Comcast speed test site.  https://speedtest.xfinity.com/

I also recommend doing speed test on multiple devices.  Sometimes the slow speed could be from the Wi-Fi connection.  

While the modem isn’t going to degrade over time the cables in your network certainly could. The rating of your Ethernet cables also have an effect on speed. If any of the cables used in your home network are cat5 then it would limit the speed of your network. Cat 5e has less interference and supports a higher bandwidth as well as the higher speeds. If you plan on one day having internet speeds in the Gigabits you should use Cat6 cables. I would first check all of the Ethernet cables you are using, inspect them for damage, verify they say at least cat 5e on the side, and that they are plugged in nicely. Then I would check the coaxial cable is firmly secured. 
 

If that still does not help I suggest the issue may lie with your computer. Assuming you are on windows you could hit Windows + X and go to Device Manager. Then uninstall the device related to your internet. It will reinstall itself when you scan for hardware changes. After you have removed the device and allowed it to reappear you should make sure the drivers all up to date for your system. This may require going to the website of your motherboard or manufacturer of your system. There may be some critical updates as well as recommended ones that might fix your issues. Often times these updates are not communicated to a user unless you were to check the website. It is important to keep your system updated as part of good cyber hygiene practices. Please post some new speed scores after trying these suggestions.