My guess would be geared more toward a plugin and or a theme. You can run a plugin like WordFence that monitors who and when a file was edited. This may help you track down where the fault is.
Here is what I would do:
- Remove unused plugins/themes
- Update all you plugins to the latest version
- Update your theme(s) to the latest version
- Update WP to the latest version
- Check the DB for bad code. (This will involve some knowledge of MySQL and WP tables/data structure)