The story so far. Noticed on my cable bill that I am paying for Gigabit internet. So I traded up to a Gig-capable modem. Was getting lower speeds than my old modem, according to speedtest.xfinity.com. Spent (a lot of) time on the phone with comcast's help desk. Sent modem back presuming that it was bad and went back on the old one, and was getting a solid 500-550Mb/s. Got a replacement unit. Spent even more time on the phone with comcast help desk. They finally dispatched a tech.
Tech came out, did some work inside the cable box, and ran a new temporary cable to my the drop on the back of my house. Speeds were still mostly 200-400 Mb/s, peaking at about 600 on comcast's speedtest site.
Called comcast for the third time. They said they still saw problems on the line. Dispatched another tech, who checked everything and said it was all good. We discussed all the bad speedtests. He said to use ookla's speedtest.net instead of comcast's. Sure enough, ookla has consistently hit 800-925Mb/s almost every time I have checked.
So all of these gyrations and thrashing about on the Gb modems was because speedtest.xfinity.com sucks. I thought I was doing myself a favor running it "in house" instead of going outside the comcast network. Who knew? they came out and buried the new cable today, so at least I got a new run out of the deal. And there may have been a problem in the box or with the cable, but most of the problem was on their backend.