As my issue (which feels similarish) is intermittent it is impossible to know if I have solved it or not yet.

I have changed my Outlook 2013 settings so that instead of replying within the same window as the message (a new bit of functionality for 2013?) I have used, File, Options, Mail and ticked open replies and forwards in a new window.

So far I have not had the issue occurring any more and that is the only change I have made. However, as it is intermittent I might well be posting back again saying that I spoke to soon!

Hope that helps a bit,

Richard