Can you help me diagnose an internet connectivity problem?

I’d really appreciate some input here. I can barely get online (I often get DNS error messages) and when I do, pages load like it’s 1996. This has been happening for the past week or so. It doesn’t seem to correspond with peak usage times or anything, it’s just random. I’m used to slow internet service, because there’s one company with a monopoly on it in my area, so they can get away with crappy service, but this is just unprecedented.

-Yes, I’ve turned off my router and turned it back on again. Numerous times. It might speed up the connection for a few minutes but then it goes glacial again.
-Problem occurs with both browsers I have, Chrome and Safari.
-Problem seems to occur most often when I try to load google and gmail!

I’ll be happy to provide more details if that would help. I would really appreciate any thoughts on this.