Let's take a look at what is starting up with the computer. Open system resources and select users and groups. Choose the login items tab and see if there's something starting with the computer. You can remove everything from that list if you want to - these are just programs that start when Sierra does.
If it's not a startup item, let's get a copy of Malwarebytes (Malwarebytes.org) and run the scan to see if this is a piece of adware or malware.
You can also start the computer up in safe boot (hold down the shift key when starting up) which will make the computer start with no third-party apps. If that solves the issues, you just need to disable the third party apps and add them back one by one until the issue re-occurs.