LISTBOX Selections not working...

PERL: AS 5.12
TCL 8.5.9
Windows XP SP3

Using Tkx...

I have a situation where I bring up a window with a listbox contained in said window. On occasion which I can reproduce, but not consistently the listbox will display, contents updated, but I cannot select any items from the listbox. The code can mark an item active, but it will not mark the selection set or mouse click of an item as selected. I can see the selection highlight flash. Again this happens sporadically. When I cannot select items, I can click on the scroll bar to the listbox and the selection works!

What conditions from a Tkx/Tcl/Tk perspective would prevent a listbox from allowing selections to be made? What commands could I use to detect a "grab" somewhere in the environment that could be interferring?

Any advice is appreciated.

gary