The stepper will do that if it encounters a seek error. Its also possible the head amplifier is bad, going bad, or the head itself is bad. (rare)

I used to have an MFM miniscribe that did the same exact thing.stepper ran back and forth over and over again. I took the PCB off, and swapped it with my SCSI miniscribe. Same mechanics, different PCB.

Problem carried over with the MFM drive board, which told me the head amplifier is actually on the board itself.