I am having this issue, as well. iMac 27" (late 2013), running Sierra 10.12.4 with an NVIDIA GeForce GTX with 2GB of VRAM and 32 GB of regular RAM. The restarts usually happened when I'm teleporting, but have just started happening at odd times during various points in the game. The last one just happened as I was heading into Whitebrim.

My bluetooth speakers are off (as they were causing the game to have kernel panics & the entire system to reboot when FFXIV ran); the only bluetooth I have running is my mouse. These restarts are really getting annoying and are interfering with gameplay. Interesting thing: my husband is also running an iMac, a year older than my machine, on the same OS version, and has had no problems.