Many users have been reporting a restart issue with the F3 family (F3, F3HR, D2 Bravo) while in an activity.
Cause: Watch memory is full and the current activity is causing the watch to restart.
1.) Keep the activity history cleared. First Sync the watch. Then from the watch, Long press the up-button at the Main Menu> History> Options> Delete All Activities. Do this Weekly/Monthly.
2.) Keep one CIQ app slot free to allow more memory for the activity FIT data.
This will keep your F3/F3hr running smoothly.