It turns out that the event is raised by a background worker thread, so the code runs in that workspace where it is illegal to do any UI work. The fact that some of the UI work is fine while some causes the hanging is odd, but at least we know the cause now. The officially sactioned work-around is to use BeginInvoke(), but the timer work-around does the job as well. I'll try out BeginInvoke() at some point when there is more time to play with it.

Thanks everyone!
Mike