I can't install 2016.20 onto my Nuvi 56 for a few days, I will report back when I have checked it out. In the mean time, I would try a proper power down of the unit (rather than placing it in standby), with the unit on, press and hold the power switch until 'Turn Off the Device Cancel / Off' is displayed,( this can take between 3 and 10 seconds), select Off, the unit will then do a full power down. When turning the unit back on from this state it may require a longer than normal press of the power button
If the above doesn't help the restarting, you can try a master reset, but BEWARE, this will set the unit back to factory defaults and delete your Favourites so make sure they are backed up somewhere.
1) Place the unit in full power off as shown above
2) From off touch + hold lower right hand corner of screen
3) Briefly press power button until logo is displayed
4) Continue to hold lower right hand corner for 30 seconds
5) When Garmin logo disappears release lower right 'touch'
6) Logo re-appears, with copyright information
7) Re-apply touch to lower right hand corner
8) "erase all user data No / Yes" message is displayed
9) Select Yes