What version of Premiere Elements are you using and on what computer operating system is it running?
I believe that you are referring to the scroll bar at the bottom of the Timeline Expert view (current version Premiere Elements 11). Generally what you describe sounds "normal".
Typically I can place the mouse cursor over that dark sliding bar as shown in the screenshot to get the movement of the Timeline from left to right and right to left. Please note the relationship of the size of bar to which I think you are referring as it relates to expanding the Timeline using the -+ slider above the Timeline. Scrolling of the Timeline can also be done by continuous clicking on the triangles at either end of that bottom area.
The above have been my observations on the matter that you present.
Thanks AT. I am running PE 11 using Windows 7 64 bit. I am attempting to scroll the timeline by repeated clicking on the triangle. However, when I release the mouse button and then later return to the the triangle, the timeline scrolls when the mouse cursor is only hovering over the triangle without clicking. Also, it will often scroll simply by moving the mouse cursor to the extreme right or left of the screen even when I don't want the timeline to scroll. It's as if it has a mind of its own and is not waiting for my click command.