Suddenly I am getting the following message when I try to close a workbook:

"Your formula contains an invalid external reference to a worksheet.
Verify that the path, workbook, and range name or cell reference are
correct, and try again"

The mysterious thing is that it does not happen consistently and that, after
I click OK after the above message, I can still save the file.

What might be the cause of this error message and can the "invalid
reference" be tracked down using one of the utility add-ins such as J.
Walkenbach's PUP?