What is causing the appearance of inverted commas in the middle of words in index entries in Word?

I have been compiling index entries with subentries in a largish (1. 73 MB) document in the latest version of Word, on which I and others have been working from a long time (ca. two years).  Index entries involve encasing in inverted commas the word(s) to be indexed and any switch code that follows, while also putting in other bits and pieces on either side of the word(s) to be indexed.  Suddenly, about a month ago, inverted commas started popping up in the middle of indexed words in index  entries and in switch codes, whether the inverted comma was either simply inserted or actually replaced letters in indexed word or switch code, thus

Paus. 6. 3. 7{ XE "Paus. ”. 3.“7" \f "literary sources"} ; Paus. 8. 45. 4{ XE "Paus. 8” 45.“4" \f "literary sources"}

This not the actual text, which I cannot copy and paste (and for some reason, I can’t upload a screenshot, either) , but it makes the point and I’ve put in red the inverted commas that shouldn’t be there (A number in the indexed entry, a full stop and a space have been replaced by an inverted comma). 

The corruption does not seem to have spread much outside index entries, although there is the occasional messing up of superscript in dates.  For example, ‘in the 6th century’ (where ‘th’ should be superscript), becomes ‘in th(superscript)e 6(superscript)th century’.  There are also other, but very rare disturbances in the text.

In the hope of avoiding the corruption spreading, I have been producing a copy of the file with a new name every 45’ or so and working from this new copy, although this does not seem to cure the problem, as the corruption comes back after two or three saves (to my computer, to an external drive, a stick and to the cloud).  The corruption is not spread evenly throughout the file and seems to crop up mostly in the third quarter.

This is a dreadful time-sucker.  It takes six hours to put it all right whenever it happens.  Life is too short for this and any help will be appreciated.

Thanks

Andrew Farrington