My 635 has recently been switching itself off and on again without any input from me.

[WPCentral Question]

The first thing I know about it is when it vibrates and Nokia shows up when it re-starts - battery OK - nothing running - phone on Stand-by when it happens

 

[ven07]

How long have you had the phone?

 

[RumoredNow]

You could try a soft reset... Counter intuitive as you will reboot a phone to defeat unwanted reboots, but it clears out system memory.

Hold the Volume Down + Power buttons until the phone vibrates and reboots (about 10 seconds). You do not lose data.

If that doesn't work... Make a backup and reset the phone. Use the second tool in this help thread (Lumia Software Recovery): FAQ - How can I recover/reset/restore my phone software? - Microsoft - USA This one will cause loss of data.