Significant number of connection drops last 2 days + reduced speed

I've been on and off the phone with Comcast "technical" support for the better part of two days trying to understand a mutlitude of issues that started Thursday.

The first:  my connection drops consistently every ~30 seconds

The second:  I'm maxing out at 10MBps down despite paying for 50MBps

Any person that I've been able to get on the phone has been completely unhelpful (try as they might) and throws my equipment back in my face (because it must be my equipment and must have no idea what I'm doing).

From the top...I use my own modem (SB 6121) and router.  I've been using the same combination for the better part of 3 years without issue.  On Thursday, the connection started dropping consistently.  Looking at the modem signal:

Everything looks fairly legitimate here.  Downstream SNR is within the normal range.  Downstream power looks good.  Upstream power looks normal.  

Despite this, I went out and bought a brand spaking new SB6121 and it showed results near identical to what I pasted above.

Looking at the modem logs, I do see some T3 timeouts:

Time Priority Code Message

...though I'm not sure what would cause that or if it's a benign issue/red herring.

Here is the firmware info from the cable modem itself:

I've checked/replaced every single cable that I can find.  We have quad-shield RG6 that was installed in 2011 from outside directly to our panel (seen in the fairly strong signals from the modem output).  We've tried 2 different SB 6121s with 2 different routers and the problem is consistent across both.  

I initially thought it was our routers since one was fairly old (DGL-4500 from 2009) and underpowered (DIR-615) but neither of them have ever had issues, both of them work just fine with our home internet connection, so I'm having a hard time believing that it's my equipment.

One thing that did help (to some degree) was moving off of the Comcast DNS servers (75.75.75.75 and 75.75.76.76) to Google's Public DNS (8.8.8.8 and 8.8.4.4) but, again, that isn't really the problem.

I, fortuitiously, have an Archer C7 router arriving at some point today, so we'll see how that one fairs (I don't expect a different result).  I do have a tech coming out tomorrow under the threat of a $100 service charge (which is BS based upon all of the investigation that I've done), but I'd like to avoid wasting their time for an issue that I think is related to Comcast's own network and not the line itself.

To sum up, I think Comcast's technical support is absolutely awful.  I'm tired of having my time wasted by scripted steps (unplug the modem for 30 seconds, check the connections, etc).  Give me someone who knows WTF a DNS server does, what SNR means, and why -1db isn't the end of the world.

If any network engineers happen to read this, let me know if there is anything else that I can do/try on my end.