i had a similar problem at one point. someone suggested that listboxes and comboboxes are unable to store more than 255 characters, even if the list/combo is sourced from a memo field.
the solution would be to clone the recordset
and perhaps make the combo a shorter item name for the longer text (e.g., a field called 'tax' and 'description' - your description would be your >255 char text, and the 'tax' would be your PK that you'd search for the text by) you can then use the combo to find the record, and have the text appear in a text box (which can handle 255+ chars)....
don't quote me on all this, though... i'm relatively new to access. i'd say, however, you may ned to rethink how you are organising your data entry...