Thanks to all who replied to my question.
And to Rick Quatro for the prompt diagnosis!
I have been using the same script since 2005 (to produce linked callouts in a Parts Catalog).
Although nothing has changed in the script itself, the issue is apparently the behavior of FM11 in handling document display commands - which now necessitates changes the script (actually, a set of them).
Rick writes: They changed these commands to be "stack-based" so that somehow they accumulate and cause problems.
If other folks having this problem, hope this helps you too!
Mervyn