I used to have that problem as well with 4.5. I don't know what it was as it just began to work by itself. Perhaps you have too many windows open at the time. Check to see how many windows are open (ie: hypershade, hypergraph, graph editor, etc) as well as how many programs are open other than Maya.

Other than that, I've heard nothing but bad things about Maya 5.0 in that it is very buggy. Could possibly just be one of those bugs.