George Berger writes: ...
A year ago, I called and asked for upgrade of our dial-up connection to
> broadband cable.
> I had already wired our home with a two-way splitter from the Comcast
> cable entry to our home (which, by the way, provided -- and still
> provides -- excellent TV reception). I used top-line Belden RG 59 cable
> for the 20' run to our top floor den where we have our computer. > ...
FYI: RG-59 has an upper bandwidth pass of around 800-900MHz. Cable companies have long been using RG-6 for their installations which has an upper bandwidth pass up around 1.8Ghz.
I couldn't tell you what channels and ranges your cable company runs their DOCSIS channels for cable modem bandwidth, but if your area depends on channels beyond the 800MHz part, your RG-59 cable very well could have had too much attenuation in the channel allocations needed for their cable-modem setup.
Of course, this doesn't excuse the installation tech that couldn't do basic troubleshooting, but it could very well have been your interior wiring not being the proper type.