It was kind of a long shot. Glad I could help.
Matching MTA vs STA (threading model) is really important when making lots of distinct calls into any COM object. An [STAThread] directive at the top of a method is one way to be sure of threading model for every call in that method.
Looks like Thread.SetApartmentState(ApartmentState.STA) will work for a whole thread, but not apparently for thread pool threads.