Program shutsdown at a certain point. Has done this on a few pieces.

When I hit a certain section in my piece, the program shuts down. I can resume several (16 or so) measures later and continue working on that same instrument. I can sometimes do flute, oboe, bassoon completely to the end and then the clarinets an the rest will all shut down at, say, measure 60, and the program shuts off. I can start entry again (after recovery) 16 measures or so later for the other instruments, going to the end.
I experimented with copying out the measures that caused the shutdow plus a few, added measures to the end of the piece, pasted those problem measures in and was able to add the notes that were in the trigger area and copy and past them back in the original area line by line.
Then my "solution" stopped working.
Why does this happen?
Casey