Strange IE Behaviour of JQueryUI with dropdowns

I have some select boxes into the code, with no strange code.
When I include the CSS stylesheet from JQuery only in IE6/7 some
combos (not all of them) close alone when the mouse move. Sometimes.
There is no single line of JS included in the page.
When I remove the stylesheet it works as expected, without closing the
"combos" automatically. :s
Any ideas? This is killing me ! :D
Thanks,
Leonardo