Cant be found, because the DNS lookup failed?

So I've never had this problem until today. Just randomly my internet started going really slow and then no matter what website, I'd get some error and the message "[so and so website] can't be found because the DNS look up failed". This seems to happen randomly. It works for a while, then it goes really slow again, I get the error. I reset the router and everything, then it works again, but it just keeps coming back. Is there anyway I can fix this? What is causing it? I didnt change anything. This just seemed to have happened randomly.