Could be 3 things;

Your driver/Direct X setting is conflicting and or your monitor / video card may be conflicting. Depending on your system can cause that.

If your running a 32bit system rather than a 64bit system you may need to lower your specs or try running FFXIV in different modes, if Full Screen try window mode.

You could have some bad log/HD/Update pending in the background that may be restarting your PC. Could try restoration point if possible.