Problem with Buttons messing up

Hey I have a problem with some buttons Im working on in a site. I have the buttons set up right now so when you scroll over them a bar fades in behind them and when you rollout it fades away. Anyway I have about 6 of these buttons lined up horizontaly. It workes fine but when you go over the buttons quickly they lock up in the over position. What is this? I have it so it is a function in an actionscript layer.