This type of thing comes under what I call Strange, Odd and Curious Behavior, and S/O/C/B, especially when coupled with the phrase 'sometimes it does/sometimes it doesn't,' usually translates into corruption, in one form or another.
The fact that you're seeing this on multiple-controls makes it sound as if it's corruption of the Form, even worse, corruption of the entire app!
The first thing to attempt, for this type of corruption, is to create a new, blank file and import everything into it. Only takes a minute or two to create and check out, and if it doesn't work you haven't really lost anything! And, amazingly, it does frequently work!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007