You have found the reason why it is best to pick one version and stick to it. You will also see a difference in the calculation for assignments between 2007 v 2010. At some point the file will become very corrupt and you will not be able to open it. what you are seeing is the problems caused by coverting down and then re-converting up.
What you are doing will not work and it will only get worst. Sorry but you have to be using the same level.