Could you please help us with few more details to help you on this case further

Is this happening on both the network 2.4 and 5G as well.

How many devices you have connected to the modem Wi-Fi network?

Have you tried all possible basic trouble shooting by performing restart or hard reset on the modem.

Yes, it’s happening on both 2.4 and 5G.  I disconnected devices so my laptop was the only one connected to the router and the result was the same.  I powered down and restarted several times, as well as did the factory reset.  No change.  I then spent an hour with a Comcast rep over the weekend and he walked me through the same steps.  I also asked him about sending me a firmware update, as I’ve seen other posts on this site that updating from the 1.04 version (which I have) to 1.05 might help solve the issue, but he had no info on doing so.  He said there must be an issue with the device, and asked if he could send me the Comcast modem for $15/ month, which I politely declined.  Aftwerards , I disconnected the G36 and reconnected my old SBG10 and activated that to see if it might be a bad G36, and after doing the Comcast speed test the result was similar, actually a bit slower.  So I reactivated the G36, which is still showing over 900 Mbps via ethernet, but only 20-30 via WiFi, located ritht next to the device.  So at this point I’m out of ideas.

Since you are experiencing slow speeds through your Wi-Fi connection, I suggest changing the Wi-Fi channels and monitoring them for the next 24 hours.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Channel