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Called for a tech visit, and it's now fixed.
Turned out part of the problem was a bundle deal had expired and that had cut my speed some, then the new modem installation somehow got confused/lost at the Xfinity end and cut the speed limits again.
In the end I upgraded my Blast plan from Pro to 200 for $10/mo more, and I'm seeing 225 at the moment.
So there's still no explanation of all the variability I was seeing, but perhaps the old modem actually was failing, especially because I was seeing uploads twice as fast as downloads sometimes.
The online chat support wasn't usable, waited 20 hours with no connect. But the telephone support was almost no waiting, and efficient. Times change things. I hadn't wanted to call because I remembered 2+hour hold times, agony. Not today.
Gigabit service requires a DOCSIS 3.1 modem. The CM700 is a DOCSIS 3.0 model which Comcast rates at a maximum speed of 701Mbps (see https://mydeviceinfo.xfinity.com/device/netgear-cm700-557). Since the modem is not rated for gigabit service speed, it's probably getting a Performance 25/5 config file.
You need a new modem for 1000Mbps. See https://mydeviceinfo.xfinity.com/
I just want to post back with resolution, for anyone else facing the same issues.
a) Comcast did everything they possibly could to help with issues - nothing worked to improve my connectivity issues.
b) Netgear support wasn't a whole lot of help - they asked me to change channels and set RTS threshold to a lower value - but after these two steps, the next one offered was to run the device in bridge mode. Which I do not want to do! I am not paying $299 for a device to run it in bridge mode so I can use another wifi router on top of it!
c) The solution to my issues - is that I replaced the Netgear 7100V with an Arris SVG2482AC. After initializing it with Comcast and setting up my wifi network, I have not had a single issue with speed nor signal strengths, no issues with multiple devices connected to it and no error logs or event logs that indicate an underlying issue. I read somewhere - maybe it was another thread on Comcast - about another person having similar issues with his 7100V due in part to it's ability to handle multiple devices connected to it. And I would think - given my example now - that 2 strikes make for an "out". I replaced my original Netgear with another brand new one and both of the routers exhibited the same EXACT issue.
This post and thread, if someone were looking for it, should serve as notice to you, the customer - please do not purchase the Netgear C7100V nor rent it as a gateway from Comcast. It fails miserably as handling multiple devices (which is the norm in many a household now) and you will spend hours troubleshooting to no avail.
The Arris is not as good in it's web manager interface nor it's capabilities to provide for a secondary "guest" network, for example. But it does what it needs to do - set up, install, initialize and run a strong network right away. I spent too many days and hours and weekends working on getting the Netgear up and running - please do not waste your time doing so.