I am having problems with a script that I put into Excel 2007 for a combo box that hides and unhides ranges based upon the selection of the combo box. Works 7 out 10 times. When it doesn't work, it put Excel into some kind of loop that requires that I shut it down and restart. The "VBA reset" button doesn't do anything. Once I restart, the script works again. What am I doing wrong?

Private Sub ComboBox1_Change() 
    Worksheets("Plan Sheet").Range("c7").Select 
    Selection.EntireColumn.Hidden = True 
    Selection.EntireColumn.Hidden = False 
    Worksheets("Plan Sheet").Range("c7").Activate 
End Sub