Excel crashes when unhiding or hiding sheet

Hi All,

Been using Excel for a few years now but VBA for a short while so I'm not sure how everything fits together. I've noticed something with two Workbooks I've created which is rather annoying.

They both have Pivottables that have their Datasources on seperate sheets that are usually hidden. As 2007 doesn't have the slicers 2010 has I use a dropdown so the user can select the required date and then this is applied to the Pivot as a filter. The problem I have is when hiding/unhiding the data sheets Excel freezes up. I can still get in to the VB editor with Alt+F11 and save or check the code but Excel is frozen. If I kill it with Task Manager and re-open the file then it is saved with the sheet now visible but when I hide it I have the same issue again.

I would appreciate any views on why this could be happening.