A binding problem which drives me crazy

Hello,
I've been trying to solve a binding problem for a few days. You'll find the test project attached.
Do the following :
Select a member from the listbox
Type in a ZIP code (these are french ZIP, from 01000 to 98000)
Select a city in the combobox
Select another member and do the same
Now select the first member, then the second one, again, again... and see how the combobox SelectedItem is lost. Why and how to solve this ?

P.S : sorry for the french properties in my classes.