Intermittent Slow internet outages - SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing

I have Xfinity internet with a Netgear Modem and Netgear Nighthawk Wifi router. I typically get 350-450 mbps, more or less depending on the day.

For the past week I started getting outages where it slows down to 20 mbps, or even down 5 mbps. When it does this, I reset the modem and router and it goes back up to 400, then eventually drops back to below 10. Eventually it will stabilize at 400+ for the night or longer. But eventually it keeps dropping out.

Its NOT the router as I get same problem with ethernet cable from modem to laptop.

XFINITY Help Desk ran some diagnostics and saw no problems with my line (remote)

NETGEAR Help Desk had me log into modem and check event log which revealed:

SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing

So they say its a physical cable issue.

I see no immediate breaks or bends in the cable from wall to modem, but I can't see beyond that. We did have a lot of snow and ice storms in area lately (for the outside cable)

But google results say modem could be failing.

I am sure it will be expensive for Comcast to send a a tech out. A new comcast modem would be free. I am curious if its a bad cable, why am I able to get long periods of 400+ mbps?

Any thoughts on what to do next?