I've got a unit that is about 7 years old.
Recently, it started turning erratically, causing the display to
pop the Antenna not rotating message, and shutting off.

The antenna's turning, but slows about every 1 to 1.5 revolutions.

I've changed the belt, and had a good look at the unit. It appears
clean, and uncorroded.
I'd like to suspect the stepper motor, as that would be a pretty cheap
fix, but before I go there, I'd like to hear some opinions of what
else might
be causing the stepper to speed up and slow down like that.

TIA