Sunday morning, and the tech has come and gone. He seemed very good, very professional, and very willing to explain things.

He replaced all connections inside the box outside, the splitter and the cable ends. He found noise on a second line into the house, very seldom used, and disconnected that. The speed from the pole was good, in the high 20s. He ran a temporary cable from the box outside directly to my modem to eliminate the degradation on that part of the line. I'll re-route that temp cable so that I retain it as a direct run to the modem.

When all was said and done, and the dust all settled, the speed at the computer (using the Comcast rent-a-modem) was only very slightly better that before. I'll give it a day to settle in, then try the Motorola modem again.