DataGrid rows bound to the same row.

Im using WPF4 and I have a datagrid bound to to a collection of instances of a plain class.

It works fine for most of the columns but one column bound to s string property is a template column with a combobox in it which is populated from a object dataprovider.

it works fine, however, when I add more than one row and select a different item all rows gets updated with that value. I can change the other columns without this happening.

Any ideas?