I've seen this quite frequently with Cox and Spectrum services using Ubee modems. They do all kinds of strange things when they go bad. One at my parents' house would actually answer with a modem tone periodically. The only thing you can do is beg Comcast to replace their modem. Your description of the display and line key status pretty much proves that the line is going dead outside of the system.

One other thing to double check: Many of the modems have an RJ14 for lines 1/2 on the first jack and RJ11 for line 2 only on the second one. Same for lines 3/4 on the third jack. Make sure that there's nothing connected to the second pair(s) of the connections to jacks 1 and 3 on the modem. I know it's a stretch, but you need to exhaust all of your resources.

Ed Vaughn, MBSWWYPBX