People ocasionally report that Word MailMerge freezes, so you are not alone,
but I don't think it has ever been established /why/ those freezes have
occured, so maybe that's not much help.

Some questions/suggestions:
a. is this also Word 2002, or are you using one version of Word and a
different version of Access?
b. are you connecting using the default method (OLE DB in Word 2002) ?
c. how do you know the query generates 600 records? Is that because the
query returns 600 records when you run it in Access, or because you can skip
to record 600 in Word MailMerge preview, or what?
d. can you modify the query criteria, for example to sequence the records
in a different way? If so, do you find that the merge hangs after 89 records
(which might suggests that the problem is related to the record count rather
than content) or that the merge hangs on a specific record ( in which maybe
there is something unusual about the content)
e. does the query return exactly the columns you need for the merge, or
lots of additional columns that you do not need? If it returns more columns
than you need, can you modify the query to reduce the column count?
f. does the query return memo fields? If so, how many per record?

I don't know if the above questions will lead anywhere but they are the sort
of things I would start looking at.

Peter Jamieson