Intermittant Internet connection

I had (have) an AT&T 2Wire gateway modem. Two days ago I began to get DNS error windows when trying to access the Internet. This was intermittant as sometimes I would get online and other times it would just disconnect from the Internet. Calling AT&T I and the tech on the other end decided it was the modem. I happened to have a 2nd 2Wire AT&T modem on hand, installed & configured it and began having the same problems.

This morning isn't as bad and speed checks are indicating that all is OK—sort of. I am still having Page Not Found errors with windows saying to chekc with administrator or ISP. Is it a fluke that 2 modems would be experiencing the same problems? Ping Host name and IP addresses (Google) has been doing OK this morning, but from time to time, as I open up another tab to go to another website, I get the error message and it doesn't connect right away. Then without me doing much of anything, it will connect.

This happens on several of my computers (2 Windows 7 laptops and a Vista Business desktop).

The 2nd modem was purchased at a yard sale (I know, I know) so maybe it has the same problems. I don't know.

Just looking for some suggestions before I run out and buy a new modem/router. I have DSL (obviously) and have three swithes on the network.

Thanks