This problem started on moving to Word 2010 running on Windows 7 (64-bit).
I work as a technical editor and most of my day comprises editing engineering reports.
When using the mouse, there is normally an I-beam cursor with which I can select text. Periodically, the I-beam is replaced by an arrow cursor which will not allow me to select anything. This happens, quite randomly, every ten to fifteen minutes or thereabouts. The only way I have found to get the I-beam back is to scroll to another page (where the I-beam mysteriously returns) click in this page and then scroll back to my original page (hoping the I-beam doesn't disappear on route). If the I-beam is intact when I return to my original page I am able to continue editing.

The first time this happened, I thought the file was re-paginating - but no matter how long you wait after the arrow cursor appears, the I-beam cursor will not return of its own accord.
Please can someone help as this is not only driving me crazy but costing me a lot of time...