I had constant crashes, especially when doing dungeons. I googled this and this problem seems to be linked to Nvidia graphic cards and DirectX 11.
So I went onto the log on screen of FFXIV, the page where you have to sign in.

On top you can see the config button. Go in there and set DirectX support off !!

I had not a single crash since.

I hope it does help a lot of you guys, without having to change all your pc parts :-).