Tracy,
We have also seen this behavior in comboboxes. The problems seem to start when are loaded with a lot of data (somewhere around 500 rows in our testing). Our workaround was to put the following into the DropDown() method of the combo.

This.RowSource = This.RowSource

This seems to fix the problem with the mouse/keyboard, but creates a new problem: The combobox does not automatically go to the row that was previously selected. Changing the rowsource resets the ListIndex property to 0. So...next we tried storing and resetting the Listbox property, as in the following

lnListIndex = This.ListIndex
This.RowSource = This.RowSource
This.ListIndex = lnListIndex

But this works only on the first dropdown...after that the combo starts dropping items and displays blank rows. So we're stuck with either having the mouse/keyboard fail to work with the combo, or forcing our user to scroll through the list from the top every time the pull the list down.

Vince