Seems to be good old-fashioned css-problem:
The top level li is 93 px high (by the link in it: 38px padding top, 16px line-height, 39px padding-bottom), the sub-navigation starts at top: 96px.
Now for the backgrounds of your problem:
The subnavigation is shown while your mouse hovers over the li (or something within the li). While you move your mouse from the top-level-li to the subnavigation-ul, your mousepointer moves over 3 px "free" space, which is not in the li. Sometimes you are fast enough so that the subnav is not invisible yet when you get to the ul (which is within the li so that the li is in hoverstate again), other times, you are not this fast, which makes the subnav disappear.
How to fix this: make the toplevel navigation links higher or move the subnav higher.