Constant DNS errors

For the past several months on certain days my internet will get really really bad and I will get DNS errors and have to restart my modem about every 20 minutes and it gets really bad. What could be causing it and would there be any solutions? I can post images of the specific problem when it happens again. and the only thing that works is unplugging my modem. Can anyone help me out?

You likely have malware which is consuming all of your upload speed. Do a virus scan.

Try OpenDNS.

You likely have malware which is consuming all of your upload speed. Do a virus scan.

Try OpenDNS.

Scanned for malware with malwarebytes and advanced system care. came up with nothing. i follwed OpenDNS instructions. not sure if it has worked.
afterwards my speeds dropped to this though...