As Steve suggested...we really need to know what kind of RowSource Type we're dealing with, here, and, I should think, exactly how you're populating the Combobox. Without knowing these things, we'd just be spinning our wheels, trying to help you.
And if you are, indeed, using a 'Value List,' rather than a Table/Query, as your RowSource, Steve's advice about using a Table as the source is dead-on. With that many items, and a list that is likely to change, over time, a Table/Query is the only approach to take.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007