Are you connecting via Wifi or Ethernet? If Wifi, it's best to switch to an Ethernet cable connection if possible. That would allow you to determine whether the problem is the Wifi signal or the link between your modem or gateway and Comcast's network. Network connection problems that affect both Ethernet and Wifi devices are often due to poor coax connections or damaged coax cable, usually in or near your home.
If you want to troubleshoot this yourself, please see Internet Troubleshooting Tips. If you still need help, please post your Internet plan speed and the following information from your modem or gateway:
Performance Pro+ Internet - 200/5
iMac is hard-wired to Nest WiFi, which is hard-wired to modem.
Modem: MOTOROLA 16x4 Cable Modem, Model MB7420, 686 Mbps DOCSIS 3.0
|Channel||Lock Status||Modulation||Channel ID||Freq. (MHz)||Pwr (dBmV)||SNR (dB)||Corrected||Uncorrected|
|Channel||Lock Status||Channel Type||Channel ID||Symb. Rate (Ksym/sec)||Freq. (MHz)||Pwr (dBmV)|
Most recent event log entries (most recent prior to these dated 11/9/2020). I suspect the 12:28 entry was when I unplugged the modem & plugged it back in to see if that would help.
|Tue Feb 09 12:28:40 2021||Critical (3)||Resetting the cable modem due to docsDevResetNow|
|Time Not Established||Critical (3)||No Ranging Response received - T3 time-out;CM-MAC=00:40:36:43:a3:b0;CMTS-MAC=00:01:5c:9e:54:5a;CM-QOS=1.0;CM-VER=3.0;|
|Time Not Established||Notice (6)||Honoring MDD; IP provisioning mode = IPv6|
@mjm54 wrote: Performance Pro+ Internet - 200/5 ... MOTOROLA 16x4 Cable Modem, Model MB7420 ...
"docsDevResetNow" is a restart command from Comcast to the modem, sometimes initiated by them, sometimes by you using the "reboot" function in the My Account app or web pages. Also, be aware that Comcast rates the MB7420 only for speeds "Up to 374 Mbps", not 686 (see https://www.xfinity.com/support/devices/).
The downstream power levels and SNR are out of spec (too low), and at least one upstream channel is borderline too high. Note that there are other readings that we don't have access to because the modem doesn't report them.
If you can't find the problem or you'd rather have Comcast take care of it, 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 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.
Thanks for the information. I'm going to check the coax cable when I get a chance. It was in the house when we moved in nearly 3 years ago and the outlet has been behind a large bookcase since then. It's a quite different coax outlet than any I've seen before. There's a rather large white box with a few inches of cable protruding from it that has a longer coax cable attached to it, that is then attached to the modem. The cable attached to the modem will be easy to replace but I'm not sure about the cable coming from that wall-mounted box.
I replaced the unnecessarily long cable left by the previous owner and reset both the modem and the Nest Wifi, and speedtest indicates an upload speed of 3.97mbps, twice as fast as previously.
Thanks again for your input.