Dynamic text change on Rollover

Hi all.
I have just this weekend made the move from Flash MX to Flash 8. I am doing something that usuallly works for MX, but not here (I take it its due to AS2, which I dont have experience with).
It is extremely simple (or should be).
I have a button, and a blank dyn text field (instance name “txt_title”).
The actions on the button are :

on (rollover) {
_root.txt_title.text=“This is image 01”;
}
on (rollout) {
_root.txt_title.text="";
}

All well and good, but the desired text only appears for a split seond before dissappearing, where as it should surely only dissappear on rollout ???

D