Last Friday I began a model on SketchUp, everything worked great all day. Monday and Today, I opened SketchUp to continue working on this model. For no apparent reason the screen will sort of freeze up. The Icon for the last tool used stays as the mouse cursor and can move around on the screen but is unable to select anything, the keyboard doesn't work, and I have to unplug the computer to move on. This has happened using several different tools, and at fairly random intervals, anywhere from a couple minutes to a couple hours.

any ideas what is wrong and how to fix this?