Same with me:
My FP2 phone seems to get more stable and reboots are more seldom since …
- I cleaned the phone by deleting the data in various app caches and deleting photos (first made backup and copied them).
- I disinstalled several android Apps known for bugs that cause crashes, e.g. Android Auto and Spotify.
- I disinstalled safety apps: cc cleaner and go security. (Safety Apps are an issue since they often contain spyware and/or advertising)
- I disinstalled free cell game
Result: The phone reboots less frequently and apparently only during charging.