@brs, @Matthieu_Bonaventure, before doing a “hard reset” as suggested by Robin, have you seen this topic about rebooting phones - maybe it could be the same problem perhaps?
It’s a specific problem that is occurring when a phone has the Gmail lockscreen widget activated and phones seem to reboot shortly after unlocking. As you’re experiencing a similar problem when taking call which technically unlocks the phone, I wondered if this may apply to you?
The solution is to remove the Gmail lockscreen widget for the time being - it’s been reported to support as a bug to investigate.
Have a look and see if this helps