Continuous Dropouts

I have been having a problem with internet connectivity for months. I get an average daily packet loss of 3% or more every day. I have a Raspberry Pi pinging the network every minute so I can keep track of the outages. Bypassed the house cabling and gone straight to the modem (a TG1682G) with no improvement. It looks as if the problem is related to the upstream channels - when it is down and I look at the modem 2 or more of the 4 channels are down. This modem does not have a spectrum analyzer display so that's no help. My questions are - am I correct in assuming that the upstream connectivity and bonding signal levels originate at the modem and not the network? The power levels are good when I view them, just curious. Is there any other 'undocumented' tricks of the trade for diagnosing the modem, temperature, voltage logs, etc? I have replaced the modem (some time ago) with no effect, and Comcast has been out and finds nothing wrong. The Wlan seems to always function properly and maybe the TV as well, but we generally don't watch it. Before I put in another service call and probably change the modem again (for the 3rd time), I'd like to hear any other thoughts on what I should do.