Sounds like something an anti-virus would do. I suggest that you try disabling that or at least tell it to ignore your working folder.
I've also had issues like this in the past where I've been working directly in a sychronised folder (cloud storage). If I save too frequently before the file has finished synching, I'd get errors trying to save. This happened specifically with Syncplicity, though - Dropbox doesn't have the same problem.
Lastly, I'd suggest working with a project folder, instead of a capx. You'll probably notice improved save times, as well.