Well, tried comcast phone support with no result other than hearing the script, so did more debugging by reading lots of posts for CM1000 users seeing what their levels were before/after issues, developed a list of things to check, etc.
Rehung 70' coax run in basement to move 12"+ away from AC power lines, removed 'Y' splitter that splits TV/Modem to boost signal a bit and turned off all options on linksys router to strip/simplify everything. Performace went from 200mbps to 385mbps which is a little higher than advertised plan speed. Success.
Case closed for now. Thanks to all who helped and answered PMs regarding their case as it helped me move forward with debugging.
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I just connected a new netgear CM1000V2 because my old zoom modem had to be rebooted about once a day to to lock-up (would lose connection completely). Netgear seems to have solved the loss of connectivity. I pay for 300mbps plan and comcast speed test shows my downstream is 200mbps +/-. House desktops and TVs all hardwired with home run connections to aggregating PCs<->Gig-E switch <-> router<->cblmodem. After a lot of reading-up on modem signal levels, I am questioning my downstream OFDM SNR - its 30-31 db. All other downstream channels are 35-39 SNR. Power levels look good (downstream running 0-4dbmv). Uncorrectables are very small.
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