Your response helped. I am now able to pinpoint the specific strings in the XML files that are causing the errors. However, I can't yet understand why it's causing the parser to fail.
The XML files have embedded   entities in the data elements. Every space in every data element has been rewritten to  . I can't yet associate any specific context that causes the failure but not every instance of the   fails (only 1 or 2 out of several hundred in the file). Strangely, in the specific instances where I do get a failure I can substitute a literal space and prevent the failure for just that specific instance. If I try and replace every   with a literal space the parser has even more problems.
Any ideas ??