Usual causes for ffxiv crashing ordered by chance:
1. bad luck (bad code/broken server took your client down)
2. overheated GPU (should always be less than 100°C)
3. broken configuration (delete the Final Fantasy XIV folder in your "documents/my games/" and run ffxivconfig)
4. broken installation (reinstall)
5. broken HDD (try a checkdisk)
6. bad GPU driver (try a different version)