I have had trouble since downloading OS X El Capitain
I have had trouble with internet connecting to wifi and Mac turning off, since downloading OS X El Capitain.
MacBook Pro, OS X El Capitan (10.11.2)
Start with shutdown problem
Note the time of the next unplanned shutdown. After rebooting open the Console app in Finder>Applications>Utilities and look for log entries at the time of the shutdown. Post suspicious ones here.
For wifi what does diagnostics show?
Use Wireless Diagnostics to help you resolve Wi-Fi issues on your Mac - Apple Support
This is the Mac Pro desktop forum. I requested your post be moved to the MacBook Pro laptop forum.