external Drag and drop from inside iFrame

I have a problem where the drag and drop stops working randomly when you start the drag from inside an iframe by using parent.dragStart..

Sometimes this works just fine, other times the calendar wont show a box once the mouse is over it, and wont work when you release the mouse. If you drag from the iframe around to an area in the parent window but not on the calendar it always work. It's just when you move directly from inside the iframe to the calendar that this problem occur sometimes.