There may be wireless band interference with the Powerline-wireless solution you describe, in which case the Gateway's WiFi channel selection needs to be adjusted. Because of its limited firmware you cannot disable the Gateway's wireless other than by putting it in bridge mode*, or inside of a Faraday cage.
*) One caveat with the bridge mode is the fact that resetting the Gateway (occasionally necessary to address ISP connectivity problems) will switch it back to the (default) router mode. If the final solution relies on the Gateway being in bridge mode, you'd need to call Comcast again to put it back in bridge mode since you apparently cannot do this yourself.
