For a test, does a computer hardwired directly to the router / gateway device with an ethernet cable have the same problem ?
I'm not exactly sure what you mean, but my desktop computer works just fine.
That's exactly what I wanted to know. Seems that we've narrowed it down now to a WiFi-only problem, and not a problem with the general connection to the Comcast system.
Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods.
Good luck with it !