Stack overflow using jQuery Checkbox with ui.core loaded..

hey,
anyone run into that?
i have the latest version of jquery ui but when i use it instead of
the old version, i happen to run into a weird bug
that the checkbox, on firefox it takes like 4 second before it switch
to "checked" after i click them
and in ie, it just pop up a stack overflow...
any idea?
Thanks, Elad.