Garmin Vivoactive 4 Turns Off with I start activity

Hi,

Recently, I'm having a very furstrating time with my Vivoactive 4 turning off when I start an activity. Sometimes it switches off on the first attempt to start an activity, other times the activity can be started and it turns off during a walk, run, etc. 

I've lost confidence in using the watch now and want to know what my options are?

I have auto-updates ON so I believe all of the software is up to date.