Last thursday my fritzbox suddenly started regular rebooting, maybe 5 or more times per day and it rings the phones each time it finishes the reboot so it is especially annoying at night time.
Nothing has changed, the fritzbox is in the same place it has always been the last year and a half without issue (except for the odd internet 'freeze').
Phoned snap and they are sending me a new AC adapter which I hope will fix the problem -- Just checked, and the fritzbox has a 5 year warranty which is nice.