DOCSIS3.0 works, 3.1 no longer functional

Have had Xfinity 1Gbps service in Tallahassee Florida for almost year now, using a self-activated personal SB8200 receiving a consistent 800Mbps down, 40Mbps up with no problems. About 2 weeks ago the SB8200's blue 3.1 lights changed back to green indicating only connected at 3.0 levels, and my internet speeds dropped significantly for a few days. Then I started having major Internet issues - constant ping -t requests would return positive replies 5 to 10 times in a row, then timeout 2 or 3 times, then another 5 to 10 positives, then timeout 2 or 3 times - very consistent pattern. When I could time a speed test to align with the periods of positive pings, I show download speeds of about 600Mbps and only 6Mbps up, but then 0.

Because I couldn't go w/o internet I tried activating my older DOCSIS 3.0 SB6183 and it works fine - consistent 500Mbps down/5Mbps up, no issues. Tried to re-active the SB8200, same issue as before - up then down then up then down, etc. Had Xfinity Rep come out, tested lines and all was perfect inside and outside the house. Said I should have no problems getting close to 1Gbps, like I had received for the last year with my SB8200. The Rep then said, "you're the 5th person today I've seen who had an SB8200 suddenly go out, same ping issue and everything" and told me that the issues was on the Xfinity side, that "they must have pushed an update that killed your modems" and that until Arris puts out a fix, you're out of luck. He then advised me to buy a Motorola MB8611, and said they're working fine elsewhere in my area.

Fast forward 1 weeks, installed and activated my new MB8611, SAME ISSUE! For a few seconds I can download/upload at great speeds, then nothing. Ping tests shows up then down then up again in a fairly consistent pattern. After 2 more hours on the phone with support activating each of my modems 1 at a time, it is clear that it is an issue with Xfinity - the DOCSIS 3.1 self-owned modems are just not working, but my old 3.0 works fine. We've done factory resets, pushed new boot configs, re-provisioned, etc. - the 3.1's won't come online consistently, yet the 3.0 work fine.

Any suggestions?