ExcelBanter - ComboBox makes Excel hang

Running Excel 2003 Professional, I have ComboBoxes in a user form with
ControlSource set to a named (single cell) range on one sheet, and RowSource
set to a short list referenced by a named range on another sheet. The
RowSource cells have formulas whose results vary in response to a separate
ComboBox value and it's corresponding ControlSource cell. This approach
worked perfectly for most ComboBoxes on the same form, but one hangs the
application completely when I click on the dropdown arrow (not only when the
form is loaded, but even sometimes in design view). Have to use Task Manager
to escape.

So, what the heck? I've checked and triple checked all the range names to
be sure they still exist; that the cell address isn't maybe referenced by two
different range names; that the rowsource cells' formulas return non-error
values when the cell they reference is set to the value coming from the other
ComboBox. I deleted the control and recreated it. Exported the form,
deleted and reimported it. The only solution I've found so far is to clear
the ControlSource property and use click-event code to set the cell value
instead. But that seems unnecessarily clumsy. Does anyone have a good clue?
Appreciate whatever you can suggest!