Run a virus scan. And make sure your pc is clean. Run all the updates you have available. Make sure your driver's are up to date too.
Also, an easy thing to do as well, is download something like ccleaner and run it. This clears out all your browsing stuff all at once vs going out to find it.
This will require a bit of work on your part. Download wireshark. When it asks, make sure you install winpcap as well. It's a bit raw but will show you everything going over your network from you entered data.
If you have access to the router interface, set static ips for everything you can. Then change the dhcp scope to whatever devices you have left. So if you have 3 devices you cannot static ip for whatever reason, make sure you only have 3 ips available via dhcp. Next take note of all your devices Mac addresses and go through your router to make sure thay natch the dhcp ips being handed out. Wireshark will also help you find devices that should and should not be on your network. It also could be an issue with your ISP box outside your home. Good luck.