It occurs to me that the Microsoft Jet driver does this thing where it scans the entries in the column to determine what type of data the column contains. Could it be possible that you are adding entries in the column so that that the driver flips what it thinks the column contains? Where before it might have considered the entries as text, and returned text, now it might consider the entries as numbers, and returns nothing for those entries that are not numeric...
(dunno. just thinking out loud...)
When we first encountered the problems, we tried reformatting the columns and even the whole sheet in question and for us it never goes back to a useful state. Copying the page didn't help either.
