You'll have to bite the bullet and send the flash file, because I won't be able to see what the problem is without it.

Without the code, I can only guess what your problem is... I would say you have the button enclosed in something else which is continually animating, or your hit area is an animated movieclip which has a blank frame.
If you are using a movieclip for the button, your hit/collision detection is a little out.
Until then, just some tips you should consider:
1) Text positioning... when I click on the About Us button for example, the text springs a few pixels to the left. That doesn't look good, so you should try and fix it. Try and put the text on a non-changing layer, separate to the background, which is the only part which changes. This is the same with the Portfolio button - which shifts a couple of pixels down on mouseover.
Also try and make sure that the menu text is all in exactly the same position... the Portfolio and Resume text appears to be indented by a few pixels relative to the buttons 'About Me' and 'Contact Me'.
2) Positioning of the squares... almost every single one of them is out by a few pixels. Spending some time making sure everything is perfectly aligned will give the whole site a real polish.