Just as a place to look, I have had unpredictable ID crashes when accessing files on a cloud server (granted it was Jungledisk, not Dropbox). The way I fixed it was to use their sync product so that I had a copy physically on my disk that was then synced to the cloud. I know Dropbox works like this out of the box, but there may be something about their syncing engine that is interfering with InDesign (for example, I never had issues with Illustrator or Photoshop with large files in the cloud). It may be their sync engine is too aggressive from a timing standpoint, and the file gets locked at the same time that ID is trying to write to it.
Try making a copy of the file onto your local drive and see if you have the same problem. Then put it in your dropbox folder for syncing. I know this is a pain from a workflow standpoint, but I have seen InDesign choke on cloud files. This cloud technology is great, but it still has some kinks.