My problem was finally resolved when the Comcast online support scheduled "profressional installation services" for $60. I did not feel good about paying $60 for coming to install something that has been in place and working fine for years, but my frustration level was so high I had to do something.
They ended up installing and amplifier, replacing a peice of cable coming into the house, and replacing my modem again. After these three changes, my service has stablized.
I really don't know the original source of the problem. All of my modem signal levels and error counts looked good prior to the change. None of my inside wiring was changed.
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