Re: Internet Issues Netgear CM600-100NAS Cable Modem
May be a hairbrained idea... but it is possible there could still be a legacy choke on the line somewhere. Seen it happen before... even caught it at my own house.
They used to use them to block the tv tuners from picking up the cable channels in the higher frequencies. When in place you will see a dramatic decrease in signal strength/quality around the 500-540mhz end when you look at the range in a spectral analyzer... almost like a hockey stick at 500. Signal is dropping a good bit as you move towards that lower end, and it looks like it has been periodically dropping channels below the 560-ish mark and replacing them with the higher end ones. Those channels are roughly where channels up to around 78 were broadcast back in the day.
Otherwise, double check the house for wall plates/coax cables that are not attached to anything or don't have termination caps. Open live cables are like micro antennas for LTE ingress from the likes of T-mobile and such that still use frequencies below 700mhz.
EDIT: Something to keep in mind about that signal page on the Netgears is that the error numbers do not follow the frequencies when it rethunks to bond a clearer channel--they stay bound to the left-most ID number. So if the 10th slot was initially bound to channel 10 at 543mhz, then it dropped it and bound channel 17, it is still keeping the error rate numbers from the previous 543mhz bonding. Can't "resynch" them either... can only reset all counters with a reboot.
It looks like your issues are initially starting in the first two octaves you are bonding, below 16. You could try telling it to start it's channel searches above that, like channel 17's frequency or 25's to see if it holds up better. It would trigger a sort of soft reboot while it thunks the channel lineup again, but shouldn' t clear the error rate counts... for that you would need a reboot. Not a reset to defaults, just a reboot.
Keep in mind that this strong arm test would be more a bandaid to mask the issue temporarily if it does show improvement. Those lower channels could still be in play and impact the quality of service. Once you have a tech visit and adjustments are made, you would want to reset to defaults to make sure things improve under the normal operating configuration.