It doesn't seem that the issue is limited to CGN3 modems.
I have the older SMC3DGN and I've been experiencing random disconnects across all devices (wired and wireless) over the last couple of days. The issue seems to be the same as yours. Sometimes the service drops after 20-30 minutes, and sometimes it stays up for almost 2 hours, but none the less, a disconnect does occur at some point.
I've been trying to play Destiny on the PS4 which is hardwired to the modem, and being kicked mid-match has been very annoying.