Docsis 3.1 dropping off

Hi everyone,

We have had the Gigabit x2 Plan for years now and for that time I was running my own Arris SB8200 3.1 modem. I was always able to connect to 3.1 (blue light on modem) 

Also, through out the years, we have had issues with signal strength and times of modem rebooting numerous times a day. The last time Comcast was out to troubleshoot - which was months ago, they installed a powered signal booster in the garage near where the cable comes in from the street.

Even with this I had many months of blue, 3.1 docsis connection on my 8200.....until.....

Very recently, approximately 3 days ago, I noticed the modem switching back and forth  from 3.1 (blue) to 3.0 (yellow) connections. 

It then started to have slight service interruptions (the bottom globe light on the 8200 would flicker)

Then about a day or so ago, the modem would never connect to the 3.1 signal - not even intermittently.

I went to the Xfinity store to see if getting a gateway would help

Last night I got an xb8 - (running in bridge mode)  - I noticed that it did not connect to 3.1. (all channels seem to connect to 256 QAM)

I have an appointment scheduled for tomorrow.

But, I know these kinds of issues are hard to find, especially because usually my connection signals, strength, etc usually test fine while the service person is here.

What is usually the cause of the 3.1 drop outs?  is it the signal booster? I haven't tried connecting with out that plugged in.

Or is this usually a situation of something beyond the cables/equipment in the home?

Thanks