Mouse wheel scrolls the page down then up, or up then down

It's dust.

No, really: it happened to me many, many times, and the cause so far has always been dust inside the mouse. It gets trapped in the scroll wheel and interferes with the sensors.

In order to solve it, open it and clean the wheel — or, if it's a cheap model, I recommend buying a new one.

Assuming that the amount of back-scroll is only a single click of the scroll-wheel and not half-way up or down the page, then what you describe is typical of wheel-mice, especially older ones.

If you open a wheel-mouse and look at how the scroll-wheel mechanism works, you’ll see that it has a series of bumps inside the wheel and a spring that presses against the bumps to cause the click-y feeling. Moreover, you’ll notice a series of thin slits on the face of the wheel through which a small beam of infrared light passes, and the mouse detects the interruptions as the wheel rotating.

The problem is that the bumps inside the wheel (“resting positions”) are not always perfectly aligned with the slits. This can be exacerbated with age due to wear. As such, when the wheel is rotated, the mouse things it has stopped scrolling, but when you release the wheel, it settles in the bump, causing a slight rotation forward or backward.

There’s not a whole lot that can be done about it, but one option is to open the mouse and remove the spring altogether (the one with an end inside the wheel, not the one holding the wheel up). Personally, I find it better because then the mouse has no clicking at all and feels smoother. In fact, it is quite desirable because excessive clicking with some mice can create a big headache due to all the vibrations. The problem of course is that if the wheel is too loose, then it may spin too freely and cause its own erratic scrolling issue.

I just switched the number of scroll lines from the default (3) to 5 lines and this seems to have stabilized the erratic behavior.