After some jawing about what to do next, the tech offered that he had an older model of SpeedStream modem on his truck -- he'd removed it from a customer last week after upgrading them to the newer model. He trotted out to his truck and got the modem. We slapped it on the line and ... *zoom* ...it worked like a champ. Same clean operation and solid performance as NetPlex's test modem. Odd. Sometimes an older piece of equipment is the thing. Of course, this older model is discontinued, so I'd better pray that it doesn't break any time soon!
With the line problems resolved I've started migrating all of my sites over to the new servers. HouseofHum, AVSrinivasn, ElizabethBear, BackBench, GPIP, GlastonburyRiverfront and PeriplusBooks are all moved. The next step will be getting the DNS servers on these two boxes squared away so I can start migrating zones onto them.
Ah, the joy of do it yourself projects!