Yeah, I think like manoloz said it might either be my driver or video card (I'm using seven year old equipment).

What's happening, for example, if I opt 'faces' and use the rectangle to choose only the faces on the upper left half of a cube, once the selection has been set, and I scroll around the model, some random vertices will also be accidentally selected or some random vertices, that were meant to be captured in my selection, will be left out.

Sorry for the grammar - I'm typing like I talk. LOL.