Buy your own modem You can get a Docsis 3.0 modem at a decent price! Motorola and Zoom sell them. Don't rent a modem, its a pure old cash cow. They rent out old models.
I had RCN too and had problems with them sometimes too, especially Internet.
The best you can do to relieve this situation is to get a back up connection (ATT DSL - which is cheap now) or 4G (Wimax) from Clear.
Just make sure you get a dual wan router or if you get another wireless router for wi-fi connection make sure they do not overlap on the same channel. The non-overlaping channels are 1,6,11m (talking about g standard, dunno about n).
You can always go with a long twisted pair Ethernet cable