Losing ability to select clips in Edit window

[kheftel]

Hi,
I am running Pro Tools LE 7.1 on a Mac G5 DP 2.0GHz. I am having a weird problem where sometimes I click on a clip in the edit window with the multi-tool (the one that does selection, crossfades, moving clips, etc), and it selects it for a moment, but when I let the mouse button go, it deselects it. Also, if I use the tool to try and select a portion of a clip by dragging the selection doesn't stay. I'm finding I have to shift-click to select ANYthing. I assume I'm missing some setting or accidentally hitting a keyboard key that changes some setting to make this happen. Any ideas what it might be? Do I need to provide more information? Thanks in advance,
Kawika