slider with scriptaculous

[Pif]

Hello, I've used the double slider with scriptaculous.

There a bug I cannot resolve : when I drag and drop right handle, the
handle move and the value change only if my move is above nearly 30
px.
This does not occur with left handle.

May be it's linked with the size of the image ?

Is it a famous bug ? Is there any solution ?

Thanks a lot.