Why do my picture files start scrolling like mad, all by themselves?

I go into my picture files, and begin using the scroll bar to the right to go through them. Without warning, and for no reason, it starts auto-scrolling through them like mad. The picture I want flashes by, and I have to back out and start scrolling back up, and hope I get to my picture before it goes apeshit on me again. It does it even if I use the little arrow ^ at the bottom of the bar to go through one at a time. It does it even if my cursor just happens to be hovering on the scroll bar, even if I’ve released the mouse. It is SO annoying and SO tricky. Why is it doing that and how do I make it STOP!?