I'd check the mouse first. Got a spare one?
A quick click leaves the image zoomed; a slightly longer one doesn't. It could be that you sometimes hold the click too long, or your mouse button might be a little sticky and sometimes doesn't release right away.
Brian, I resolved not only the zoom problem but also a lot of 'stuttering' and unresponsiveness of the sliders in the Develop Module by cleaning up my two optical mouses (mice?!) (google it for specific instructions). Hope this works for you, too.