I just searched for this problem on the forum and found this issue: https://sourceforge.net/p/texstudio/bugs/136/?page=1
But then it says it is closed, although I have a very similar problem as the OP. When I type, sometimes, which seem to be totally random, the accents don't appear. For exemple, I can sometimes write "la tête" on the first time, with the ê being correctly typed. But few second later, I might type "la tête" but TexStudio will enter "la tete". I have the same problem for the ` on the è.
Is there a setting that I must set to get this straight?