Problem: frequent spontaneous forced reboots

Hi everyone,

my Mate 7 has a tendency to do unexpected, spontaneous forced reboots every now and then. I have no idea what causes it. Recently, it happened multiple times while trying to scan my phone for viruses using the Avira app. Today, I tried to launch 3C Toolbox when it happened.

Any ideas? For info, I run firmware B137SP3 and am rooted with SuperUser.