Sharp CE-122 printer motor running too fast?

[Dave Britten]

I picked up a Radio Shack version of the Sharp CE-122 printer for my PC-1211. I've got it running with a fresh battery pack, but it seems like the printer drive motor is running too fast. If I'm reading the service manual correctly, the head should make about 10 sweeps when printing a line of text (7 pixel rows and three blank rows to advance the paper). But I'm counting roughly 19 sweeps. The printed characters end up being stretched horizontally, and some of the dots at the right end of each of the four printhead columns get wrapped down to the next pixel row. This makes me think the printer motor is running too fast, and the computer is only picking up every 2nd timing signal from the printer, as they're coming much faster than the computer is able to handle.

Anybody ever seen this before? Is it as simple as the motor drive voltage being wrong? And if so, what sort of aging component(s) might be causing it?