Folks have asked (and you've told them) where the string breaks. We don't yet know when it breaks. If it during tuning (a particular problem if you retune from standard to/from open D a lot. Or anything that uses an F# on the third string) then I'd recommend a bit of pencil "lead" (graphite, really) in the nut. That will help reduce the friction problem. If it's during playing, then... see previous answers.

--
Hamish