Okay this makes no sense at all.
I blew the mouse out with dry compressed air. I then plugged it into my Mac Plus, which was set up and at hand, and it worked great! Tada! Problem solved, right?
So then I plug it into my Lisa.. same problem again. Goes side to side, but otherwise seeks the top of the screen and remains pegged there.
While the Lisa is still on, I swap in the Mac plus mouse. No problem... everything works again.
I swap the Lisa mouse back.. nope.. back to the top of the screen, can only move side to side.
WTH
So then I take the Lisa mouse, plug it back into my Mac plus... and it works fine.
So the only time this Lisa mouse doesn't work is when it's plugged into an actual Lisa. And the Lisa will only work with a Mac Plus mouse.
I AM SO CONFUSED