Internet keeps dropping. Is it my modem?

Cable TV is not affected, but for about a month now (or more), we lose internet connection once a day. Now it’s happening a few times a day, sometimes back-to-back.  The cable MODEM is my own-–a Motorola SBG6580 modem/router (maybe over 5 years old). I’m looking into getting a new modem, but am curious if the modem is even the problem.

When the internet goes out, we can still see our Wi-Fi network as available for a few minutes (all modem lights are on), then all the lights on the modem go out and the connection is reset in about five minutes. 

I have not contacted Comcast about this issue, other than this post (assuming that my modem would be blamed).
No reported outages in the area, and I tried restarting the modem using the online troubleshooter and leaving it unplugged overnight.
I tried switching to a wired/ethernet connection, but I still had the problem. I removed the splitter (in the room) and connected the modem to the cable, but I still had the issue. I also connected the modem in a different room (downstairs) and still had the issue.

We don’t use much bandwidth--we use a Roku for Netflix for a few hours only, and have a computer connected most of the day for browsing/email, etc.
This may be irrelevant, but today I noticed that when my computer re-connected to the internet, it connected to the Wi-Fi extender by default instead of the modem. And the very first week I noticed the Wi-Fi going out, I remember going through the Windows troubleshooter and it said something about the DNS Server.

I’ve seen the information below included on other posts--I don’t know what it all means, but maybe it’s helpful:

********These are repeated over and over in my modem EVENT LOGS:

Mon Aug 09 18:51:37 2021        Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=[info removed];CMTS-MAC=[info removed];CM-QOS=1.1;CM-VER=3.0;

Mon Aug 09 18:48:01 2021        Critical (3) 16 consecutive T3 timeouts while trying to range on upstream channel 3;CM-MAC=[info removed];CMTS-MAC=[info removed];CM-QOS=1.1;CM-VER=3.0;

Mon Aug 09 18:48:01 2021        Critical (3) Unicast Maintenance Ranging attempted - No response - Retries exhausted;CM-MAC=[info removed];CMTS-MAC=[info removed];CM-QOS=1.1;CM-VER=3.0;

********And I usually see this too, but not as often:

Mon Aug 09 18:33:47 2021        Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;CM-MAC=[info removed];CMTS-MAC=[info removed];CM-QOS=1.1;CM-VER=3.0;

********Yesterday, I saw this ABOUT THRITY times on the Event Log:

Sun Aug 08 11:12:17 2021         Warning (5)              Dynamic Range Window violation

********And for CONNECTION STATUS, this is what I saw yesterday:
(Upstream POWER numbers in the fifties):
Upstream Bonded Channels

Channel/Lock Status/US Channel Type/Channel ID/Symbol Rate                   /Frequency              /Power

1              Locked     ATDMA                     4              5120 Ksym/sec         29200000 Hz            54.0 dBmV

2              Locked     ATDMA                     1              2560 Ksym/sec         10400000 Hz            49.9 dBmV

3              Locked     ATDMA                     2              5120 Ksym/sec         16400000 Hz            54.0 dBmV

4              Locked     ATDMA                     3              5120 Ksym/sec         22800000 Hz            54.0 dBmV

********I appreciate any input. Thanks.
Is there something I should add here, or try?
My internet just went out twice as I was finishing typing this, so hopefully I won't need to retype this!
Here are the top entries from the log now: