I've been having issues with my MB8600 locking up almost daily since comcast upgraded the lines to fiber in the neighborhood several months ago. Before that, the MB8600 ran perfectly for over 6 months. I've had multiple techs come out and change out coax and check levels with no effect. I've had the exact same expereince as was documented in this post including being told this is a known issue with comcast and the mb8600:
https://forums.xfinity.com/t5/Your-Home-Network/Xfinity-Motorola-MB8600-horrible-NEED-HELP/td-p/3158318
What I have manged to determine is that there seems to be two distinct operating levels on the line that randomly change, sometimes multiple times a day, sometime it will be constant for several days. Both power levels seem to work but when they change (+/- 7db) it causes errors and the modem to hang requiring a manual reboot as its webpage is unaccessible.
I've attached screenshots of the two levels. I've be told this shouldn't happen but the levels do work when static. I've tried just about everthing possible to figure out a soultion, but I'm very confident the problem occurs from whatever is causing the power levels to change. I have noticed this pattern since the very beginning of the issue.
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