Posting here in case anyone with a similar setup and similar problems can either shed some light, or benefit from my info once this is resolved.
I've consistently received Comcast Internet speeds between 30 and 40 Mbps since the last cable replacement in my yard some months back.
Recently, speed has been in the toilet and everything stops and buffers all the time. My speed now ranges from 250 kbps to 5 Mbps, when using Ookla and testmy. I've ran automated tests over 24 hours with testmy and the results are all within this band.
I [may or may not] have checked speed directly at the cable's point of entry to my yard and [may or may not] have received the same speeds. (Comcast has emphatically informed me that performing any diagnostics myself outside my house is breaking federal law.)
I called in and the first rep told me that my reported 250 kbps was the same as 25 Mbps, since you divide by 10 to get Mb from kb. I explained he was still off by a factor of 100, but he was not buying it. He said he would run a test, then came back and said he got 25 Mbps, "the same as you". I asked him if he actually ran a test, at first he said yes, then he went back to saying my speed was fine because the 250 kbps was what I was supposed to have because it was the same as 25 Mbps. I admit I lost patience at that point, explained the factor of 1000 conversion to the rep, and asked to speak to someone else. He put the phone down and I heard him laughing to his colleagues for a while, but he didn't come back on the line. I hung up.
I called back, and either this rep was already having a bad day or she saw something in the notes from my previous call because the first time I spoke before she was finished she let me know that I needed to wait until she was finished talking. I told her about my speed problems, and her first response was that I needed to contact Motorola about my modem. I asked her how that would help if the problem was the speed coming into my yard, and she said nothing could be done about that. I asked her what that meant, and she went into a spiel about how my SB6121 needed to be replaced, and told me I could find upgrades online at Comcast. I asked why, and she said that the SB6121 topped out at 75 Mbps, which was my current service. I asked if it was still supported, and she said yes. I told her I would be happy with "just" getting 75 Mbps. She then told me that Comcast could not control if Motorola was pushing firmware updates to my modem that were not compatible with Comcast. ? At any rate, I have checked and my modem is running version SB_KOMODO-1.0.6.16-SCM00-NOSH, which looks to be the version Comcast is looking for.
Lastly, I asked this rep how may kb were in a Mb. She put me on hold to ask her supervisor, who looked it up and gave the correct answer. She did give a disclaimer that her level of support was not as deep in the technical details, which I suppose is a reasonable answer. This obviously still does not justify the exerperience with the prior rep who refused to help further based upon his own lack of understanding.
I did press "1" prior to the calls to participate in surveys afterward, but have not received the call back on either call. If Comcast would be interested in discussing my customer experience during my phone calls any further, I would be glad to oblige with transcripts of the calls (I have an excellent memory 
I have a service appointment scheduled. I'll report back. Thanks!