I had issues on my RT-N16, also on Comcast, where the router would just slow to a crawl, even with file transfers over LAN.I can confirm this behavior on 376.44 + RT-AC56U + Comcast. Speed drops dramatically and clients cannot connect to the router or have DHCP errors after about 1.5days. A reboot fixes issue which soon returns.
Flashing .43 and doing a factory restore fixed it completely.