add-in timing problem?

I am working on a VSTO add-in using C#. I have a situation in which I am
working with large ranges of data (clearing, formatting etc.).

When I run the code normally various Excel calls will fail randomly.

If I step through in the debug window the code runs perfectly.

I am suspicious that some calls may be running asychronously and that my
code is failing because a previous call has not completed. Could this be the
case? If so is there any way to check that a previous operation has completed.

BTW: Application.Ready does not seem to help.