said by DougHo:For those interested: Tech worked several hours this afternoon. Although other stuff was done such as measuring levels at various points and replacing coax connectors, I believe the solution was exchanging DPC2100 modem and/or power supply (old power supply was a large one labeled Toshiba).
Although modem stat page says Receive Power Level increased to around 8.6dBmV, S/N improved to around 39, down is now good 11.2Mbps, up now good 1.1Mbps, with packet loss less than 0.2%
Actually there were multiple issues. I do not advise swapping modems as a solution for anything unless we determine that's what needs to be done.