Delay on rollover with Laco's tween

Hi,
I use laco’s tween class for a button color fade. A bit like on the Laco navigation itself : http://laco.wz.cz/tween/

It works, only, when the mouse moves a little too fast over the button, it delays, like the mouse didn’t went over that button, what gives a pretty annoying effect, like a bug…
this is the code I use :

[color=darkred] menu1.onRollOver = function ()
{
this.colorTo (0xFF0099, 0.1);
}
menu1.onRollOut = function ()
{
this.colorTo (0x171717, 5 );
}[/color]
[color=black][/color]
Does that look familiar to somebody?