After a week of experimenting it is clear that the InDesign CS5 cross reference program code is bad. No matter what I do, after a number of cross references (over 100 but I don't know the exact figure) have been created InDesign corrupts the cross reference data base and crashes. This corruption is permanent. Whatever I do (including deleting all existing cross references) the next time I try to create a cross reference InDesign crashes. Fortunately it appears that only cross references are affected and the corrupt documents and book can still be used.