speedtest.xfinity.com shows a download speed of 85 Mbps, which is much lower than I should be getting with my 200 Mbps plan. My cable modem is an SB6183.
My computer is connected via ethernet to a router that is connected to my cable modem, but I also get the same results when I directly connect a laptop computer to my cable modem via ethernet.
I've been through several rounds of troubleshooting with Comcast technicians and they all say that everything looks fine and that I should contact the manufacturer of my cable modem.
This is a reoccurring issue for me. The last time I ran into it was in August of 2018 according to my private messages with CCShane from Comcast and a reddit post athttps://www.reddit.com/r/Comcast_Xfinity/comments/96tjy5/942_mbps_download_speed_when_i_should_get_g.... At that time I tried several different cable modems, and they all exhibited the same behavior where I could not achieve a speed above 100 Mbps.
Around this time, Comcast sent a technician out to my house that confirmed that the signal was fine and and that I was indeed not seeing the speed that should be expected. They hooked up a Comcast-owned modem which gave me the expected speed and indicated that they could not troubleshoot any further because of this, even though I explained that it couldn't be an issue with my modem because I tried several different ones that all exhibited the same exact behavior (Internet works fine, but with a reduced speed).
Last time this occurred, the problem eventually resolved itself. I think this was around the time that Comcast gave us the free upgrade from 150 Mbps to 200 Mbps.
This may not be the root cause of your problem but it should be addressed regardless as it may be a problem just waiting to happen.
The upstream power is on the high side and it may be intermittently fluctuating even higher to out of spec levels. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.
In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test
If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.