Xfinity Says I Need To Replace My CM3000

Here's hoping that the information helps place the blame squarely on the modem or the cable.  If it does not....

Perhaps a silly question.....  If you have Xfinity install a modem, how long are you required to keep it?

I see two obvious ways to settle the question of "is it the modem or the cable?"

1. Spend over $150 to buy a new Xfinity compatible modem
   https://www.xfinity.com/support/internet/customerowned
   If the streaming problems go away and the new modem does not report the same errors as the existing modem, then the original modem was probably bad.
   If the streaming problems remain and the new modem reports the same sort of errors as the original modem, it is  probably the cable.
2. Have Xfinity install their modem (at about $15/month).
   If the streaming problems remain with the Xfinity modem, then the problem is with the cable and Xfinity needs to fix it.  
   If the streaming problems go away, then the original modem was bad and you now get to make the same decision as before. "Is it better to spend $15/month to rent an Xfiniity modem or to buy my own modem?"

I would probably go with, "install an Xfinity modem" to save the potential hassle of returning a modem if it turns out that the cable was bad.