I came across the same problem when trying to achieve similar functionality as mentioned by you. 
The problem is whenever a slider is moved it keeps setting the "selected" attribute of the select list without clearing up previously selected item. I have added a script to clear previously selected option items before setting the new one as selected. 
Look/search for the text at line number 92(..ish) of the jquery script file.
   //control original select menu
   var currSelect = jQuery('#' + thisHandle.attr('id').split('handle_')[1]);
   currSelect.find('option').eq(ui.value).attr('selected', 'selected');
and insert this code between 2 lines...
   currSelect.find('option[selected]').removeAttr('selected');
so final code should look like this...
   //control original select menu
   var currSelect = jQuery('#' + thisHandle.attr('id').split('handle_')[1]);
   currSelect.find('option[selected]').removeAttr('selected');
   currSelect.find('option').eq(ui.value).attr('selected', 'selected');
I hope it helps.
Feel free to ask any further question.