odd quirk on my MB 5,1

ok... I'm not sure this has always been happening with my mac or not, but I have noticed it recently. But I'll be, lets say surfing the net... And every so often (haven't timed it, but it's frequent... IE 4-5 times a day easily) the window I'm using is no longer the active window (for the lack of better words). What I mean is, what happens is similar to clicking in an open space on the desktop, but the finder bar (not sure what it's called) at the top doesn't change from chrome to finder if I did click on the desktop. But when this happens I'm not using (mouse is idle) or clicking the mouse on the desktop. Generally this is not really a problem unless I'm in the process of typing something.

So what is causing this to happen? What are the corrective actions?

Anyone???