Upgrade to 600 Mbps and New Modem = Unstable connection

I upgraded to 600 Mbps over the 175 plan and saved some money in the process.  I'm currently using an Arris SB6141 which is not rated to 600 so I decided to get a new modem.  I tried an Arris SB8200 and things seemed to work well at first though my speeds averaged 250 Mbps plugged in with ethernet.  After a few days, the download connection would fail and drop into the Kbps range and become unusable.  Basically the speedtests would sometimes spike to 300 Mbps and then you could watch and it dropped close to zero.  As you can imagine, this would make using a computer on the Internet impossible much less streaming anything on a Roku.  I went through troubleshooting over the phone and ended up putting the old Arris SB6141 back in and things leveled back out.  

I then decided to try a different Docsis 3.1 modem and installed a Motoroal SB8600.  It worked well at first but then started doing the same things as the Arris SB8200.  Speeds were not consistent and they would occasionally bottom out and become unusable in the Kbps range.  Xfinity says there are no appointments available for techs to come out and I've spent time on the phone and in chats trying to troubleshoot.  I had speeds in the 350+ Mbps on the Motorola modem but then it just bottomed out and I put the original Arris SB6141 back in service.

Any thoughts or ideas here?  For the time being I'm sticking with the Arris 6141 that is giving me around 130 Mbps down and 18 Mbps up consistently.  I need stability working from home more than anything at the moment so I'm not going to try to "upgrade" my modem again anytime soon.