The fact that the Expression is being used in the Control Source means that nothing should need to be done, once the Record is saved. The 'sometimes it does, sometimes it doesn't' makes me wonder if the Unbound Textbox has become corrupted.
Although we usually think of Forms or entire Databases when we speak of corruption, individual Controls, such as Comboboxes, can and do become corrupted. The test and the fix are one and the same; deleting the Control then re-creating it! If it was corrupt, you've verified the fact and solved the problem! If not, you've only lost a minute or two!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007