Flushing the DNS via ipconfig -flushdns solved the internet-connection-specific disconnects, meaning I don’t need to contact Comcast. Concerning this new advice: the DNS Proxy wasn’t enabled, fortunately. I set the 2.4Ghz band to only use 20Mhz, and so far it seems to be holding stable. I will keep tabs on it and report back tomorrow if the problem resurfaces (after seeing if 802.11n only helps in such a case); if the problem does not re-emerge, then this seems to have been the solution I needed.
EDIT: Further testing has revealed that 1.) the 20Mhz switch did not fix the problem, and 2.) switching to 802.11n exclusively makes the problem considerably worse.
EDIT 2: Performing a ping test reveals that the router itself is no longer having issues after all. The remaining issues appear to all be on Comcast’s end, as the DNS flush did not in fact resolve the connectivity issues completely.
