People occasionally report problems with large jobs. My best guess is that
the spool file may be getting too large - if so, you should be able to do
/some/ tests without wasting paper by suspending printing and seeing if the
merge completes, and how much space the spool file takes up if it does, or
increasing the amount of disk space if it doesn't). Otherwise I suppose it's
a quesiton of merging in smaller batches.

Peter Jamieson