@escue17 

This log line says Main Deco lost connection to the Internet, which could mean many things. There should be another log line, later, with OFFLINE -> ONLINE, which is when Main Deco got back Internet connection.

You can calculate duration of an outage with these two log lines. 

I also have cable modem, and adding Deco to my home network exposed an issue I had with Internet. I had Internet drops, even before Deco. It was not the Deco that caused outages, but with Deco logs I was able to see how often that happens. In my case there were daily outages of same short duration, less than 5 minutes each, happening randomly during day and night. 

After establishing what is going on I was able to convince my ISP Tech Support there is an issue, they sent technician and he replaced cable at the entry point to my house. The old cable with cracked insulation was the root cause of outages in my case.

Your case may be different. Contact your ISP Technical Support and tell them check cable signal strength and noise for cable coming to your surfboard8200. If they take your request seriously, and not just brush you off by saying all looks good from their side, they will schedule technician visit to your house.

  @Alexandre. 

Thanks and I am seeing similar thing with the drops (also before Deco) - even short duration ones - and using logs to convince Xfinity/comcast that they have an issue.  They told me it was hot out last week ;-)

ISP checked line and they said it was excellent signal but when it is up, I bet it is, but need a better tech dispatched to trouble shoot it...I will also suggest replacing the buried cable.

I just wanted to eliminate Deco from this equation as it happened with my Google wifi mesh before I upgraded.  Thanks for the note and glad your problem is solved.

  @escue17 

You need a technician who will check coax cable coming to your house. If ISP Tech Support only remotely checked cable modem status, this is not enough. Technician that came to my house did a thorough job, with cable tester device.

If technician can't find anything wrong with the cable outside the house, you'll need technician come to your house and check coax cable in your house, all the way to your cable modem.

If technician says everything is fine everywhere, have prepared list of Internet drops collected from Deco logs: time and duration, in the last 24-48 hours before technician visit. It makes very productive conversation if you can say something like this: "tonight at 2:43 am Internet went down for 2 minutes 35 seconds, and then at 4:55 am for same duration, and at 11:32 am, and just before your visit, at 1:44 pm. Random times, day and night, same duration each time. Please explain those."

You can do a bit of troubleshooting yourself. If you can, move cable modem as close to coax cable entry point to your house as possible. Reduce number of cable splitters between coax entry point and cable modem, ideally to zero, or just one splitter if you have cable TV.

I checked reviews of your cable modem, and while some people swear it is the most reliable cable modem they ever had in their life, others complain they need to bounce it often. Consider bouncing your cable modem at a convenient time and see if Internet drops dissapear for a while. If what you need to avoid Internet disconnects is bounce modem once every 24 hours, that can be automated until ISP figures permanent solution.

It can be challenging to deal with Tech Support. In your case, as you only have cable modem, I am sure they already told you it must be router (Deco, Google) fault. In my case, I had ISP modem/router, i.e., gateway, so it were clear end-to-end ISP responsibility.

If nothing helps, consider getting xfinity gateway, even if you have to pay extra monthly fee for it. Not only gateway can have cable signal status web pages you can access, but it will be harder for ISP to refuse responsibility for Internet drops if they happen with ISP gateway, which is ISP modem + ISP router.

  @Alexandre. 

Great suggestions and I tried the cable movement, tightening and trying to remove splitters (I only have 1 splitter).  Dropped 3 times tonight in rapid succession.

I have been watching the modem event log and 2 interesting things:

1. the errors prior to dropping is a "16 consecutive T3 timeouts while trying to range on upstream channels" and
2. Watching the power on 4 upstream channels are in the 49-51 range and this seems to be too high and make it vulnerable to any raise causing the modem to reset.

I think Deco is clearly off the hook.  Tried telling this to xfinity so I get the right tech person but not yet successful with them.  However, in reviewing the xfinity forums for these things indicate they should be checking cables and connections first, then some noise on the line.  Exactly as you suggested!

On the modem, this one is new anbd introduced after this problem began with the old 3.0 modem.  It may be more sensitive to the fluctuations being 3.1 but this problem did not start with this modem.

The final step is your gateway suggestion.  Or get Fios.