Word for Mac

I have an iMac11,2 with an Intel Core i3 and 8GB of memory and am using Word for Mac v16.26. It seemed to work fine for the first year or so I was using Office, but a few months ago it started regularly freezing my computer. After 20secs or so, the computer makes several loud harsh beeping sounds, then turns off and restarts. No warnings, error messages or any other text on the screen either

before or after the restart.

This only happens with Word and has never happened while using Firefox, Chrome, OpenOffice or anything else.

I followed advice from another post about emptying the autorecovery data etc, but that hasn't solved the problem. It's not a storage problem, as there's 800GB free on the hard drive.