What version of Word are you using.
This could be an indication that this document is starting to corrupt.
Do you get the same crash if you modify this style in another document based
on the same template?
If this is the case the following could save your document before it is too
late.
1. Copy the whole document, except for the last Paragraph Mark into a new
blank document. Either press the ¶ icon or Ctrl+Shft+8 to toggle the view of
the non-printing characters.
2. Or if you have 2003, close this document and reopen if via the Open
dialog and use the Open and Repair option from the down-arrow next to Open
button.
Hopefully either of these can save your document.
If the crash occurs in different document based on the same template, maybe
try option 1.
Hope this helps
DeanH
