Mail Merge problem

My client is having trouble with Mail Merge documents in Word. She has just one source database in Access and about 10 different documents of various types, label, letters, and the like. A few different things happen when she tries to open one of the Word docs. Occassionally it will tell her that it can't find the data source. You can manually set the data source, and it may work one or two times, but it may not work again later, or it will stop working on a different document. She also randomly gets other error messages talking about something called DDE.