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.
Solution:
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.