corrupted message in draft

I am running Mavericks 10.9.5 on a Macbook Air 2012.

Mail Version 7.3 (1878.6)

In the Mail application, I frequently prepare messages and put them in the Draft folder.

Very often, the mssages are corrupted when I go back. Parts of the text are missing, "=" characters appearing everywhere and sometimes the text of a previously Sent message is appearing.

I think (but I am not sure) that it started after upgrading to 10.9.5.