I have a spreadsheet with several macros. Generally, when previously using Excel 2007, a user clicks a button and everything works as expected (calculations, some email sending & file I/O). Typically, the expected run-time is about 90 seconds. The spreadsheet is a xlsm file created with Excel 2007.

With Excel 2010 however, the same user process results in a non-responsive excel and forces us to kill excel from the task manager.

Some note that I have gathered so far in trying to debug this issue:

1. When monitoring CPU usage, it seems that Excel does start the macro. CPU usage increases as expected to about 47% for a few seconds. Excel.exe than drops to 0% usage and I now have a non-responsive Excel (even after 1 hour).
2. If I set debug break points across modules and different functions and step through the code (after clicking the button) , the process works as expected albeit much slower. To add, there were no exceptions.

I am at a complete loss as to what the issue may be. I initially thought it may be the add in that is being used but that was debunked by point 2.

This seems to be a very odd situation. I can provide more information if required, but I'm at wits end about the root cause could be. I need help in diagnosing and resolving this issue.