SB8200 drops DOCSIS3.1 off and on throughout the day

...  900down/25up plan ... SB8200 ... loosing it's 3.1 connection ...

Many of your downstream channel power levels are out of spec (too low), and the (presumably OFDM) 957 MHz channel power level is, as the Brits sometimes say, "right out!". Network connection problems like this are often due to poor coax connections or damaged coax cable, usually in or near your home. Running the cable through a surge protector, a defective splitter, or too many splitters can cause signal problems as well. If there is an amplifier in the line make sure it's getting power. 

If you want to troubleshoot this yourself, please see Internet Troubleshooting Tips. If you can't find the problem or you'd rather have Comcast take care of it and an employee does not respond to your message here, call them at the phone number on your bill or 1-800-Comcast, or use one of the options on https://www.xfinity.com/support/contact-us/. It's not likely they can fix the problem remotely. If not, insist they send a tech out to identify the cause and correct it.

If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit (approx $100) unless you have their Service Protection Plan ( https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast rental device or anything outside your home you shouldn't be charged.

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For some reason I don't see the ability to attach a photo so I am sharing from google docs.

So here are the connection rates from tonight... connected to DOCSIS3.0

https://drive.google.com/file/d/16_RIiKDs3-cxttEGjdnH-sk9mhTA0DKo/view?usp=drive_link

And then after a reboot connected very briefly to DOCSIS3.1 (less than 2 minutes tonight before falling back to 3.0)

https://drive.google.com/file/d/1wOG-gHEjPVT-f_9X_Sv2D1-_K9LM2hz-/view?usp=drive_link

Bruce, thanks for your insight.  Just called and I am on the waitlist for as early as tomorrow at 10AM, or 10AM Monday.  I have a confirmed appointment for Tuesday at 8AM if nothing opens Sunday or Monday. 

Didn't even fight me or try to take me through troubleshooting or rebooting the modem.  Thanks to you confirming what I thought I was confident to say I had sever downstream channels out of spec and that I have observed my modem going in and out of docsis 3.1 mode several times a day, which has cascading performance effects. 

Never before have I had them offer to send a tech the next day.  

I was looking at the line outside my house for any damage and while out there I was reminded of two recent changes. 

1. They put in a new telephone pole about a year ago and just over a month ago they came through and moved all the non power lines to the new pole and took out the old pole.  Wonder if they didn't twist down my coax all the way at the pole.  

2. I have a new neighbor moved in about 2 months ago.  House has been vacant for a year, previous owners had satellite, I noticed a new coax line running from the junction box to their house.  Would one new house on the circuit be enough to cause my low levels?

Either way thanks again Bruce for confirming what I wasn't confident enough to call with.   I'll post a follow up after the tech comes.

... moved all the non power lines to the new pole ... new neighbor ... new coax line ...

I'd say you are right to wonder if those changes have something to do with the signal problems you are having.

... I'll post a follow up after the tech comes.

Please do! We always wonder how these things turn out.

Good luck!

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Update... Xfinity tech came out today.   Nice guy was well informed and had already seen my low readings and had a plan of attack.  He replaced all my coax connectors (at the modem, at the pole, at the house) to make sure we were not getting leaks there. Took one extra step and went back up the pole and moved my connect to another port, nothing changed.  Levels all read the same.  Said the issue is definitely not localized to my house. The problem is in the line that feeds the house or in the junction box that my house is connected too.  Interestingly the neighbors came out when he saw us said he has been having issues too. 

So the tech is escalating it up as this fix is more than he is trained or authorized to address. 

What seems odd to me is Xfinity must be able to monitor/level these power thresholds for the neighborhood?  If this was a neighborhood level issue, wouldn't they see it, get alerted to it?

Also after monitoring things through the day my power levels actually look worse?   Even more interesting is the number of uncorrectable that didn't exist before vs exist before.   I chatted with a support today to be sure the tech escalated the issue up and see if I could get an estimate on time frame.   This is what I was told: "our higher team is checking on this like it can take more time because they going to take a on your network it going to take at least 5 to 7 working days because it will check weekly basis"    I got a ticket number so I guess I just wait 5 to 7 days and bug them again.  

...  I got a ticket number so I guess I just wait 5 to 7 days and bug them again.

You're kinder than I would be. Comcast is not exactly famous for keeping their promises, nor for doing so in a expeditious manner. I'd be jumping up and down on them hollering "FIX MY . . . STUFF!". But maybe they'll surprise us. Again, good luck!

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Finally!  After two more tech visits!  I am fixed.   They sent a second tech out (Contractor for Comcast) after changing all the coax ends didn't work.  This one came with some kind of meter device and could see my low ends very clearly.  He replaced the line in my house (I only have two lines, pole to house, house to modem).  He didn't think it would help, but he wanted to try it first as comcast will charge for the pole to house line.   My numbers went up but still not strong enough for a docsis 3.1 connection.    So he said that was all he could do with out charging me for the pole to house line.  I needed to go so I didn't argue with him and sent him on his way.  

Contacted comcast again via chat to confirm if the line from the pole to house was my responsibility, they actually said no.  I asked can you send someone out to replace it (really scratching my head as this could of been solved in one service call).  They said yes and scheduled someone next day.   Tech showed up.. took all of 20-30 minutes and I was back up and running and have been fine for 48 hours.  The problem was my wire had been riddled with what look like tears... like a squirrel was grinding down their teeth on my wire!

So for anyone else reading this forum having the same issues... be more persistence that I am... make your sure your power levels are between -10 and +10  (Thanks Bruce for your help there) and based on my experience don't let the contractors tell you the pole to house is your responsibility. 

Look at the difference in my power and SNR numbers now, compared to 20 days ago!