Data Validation slowing down workbook
 

Hey, I have a problem when using data validation on some
large excel workbooks (a lot of formulas & VBA code).
Well, let me explain the basic design of the sheets.
There is a bunch of data that can be sorted and subtotaled
through a button(vba code attached). There is also
another button that adds an item to a range that is used
in a data validation drop down list.

the sorts subtotalling button usually runs very quick, but
if I add an item to the list, it makes the subtotalling
very slow. It just keeps getting worse as I add more
things to the list.

However, if I save the workbook and re-open, it runs fast
again... It seems like excel is holding on to something in
memory, but I checked and re-checked the code and
everything is being released.

any clues?? thanks1