PLEASE HELP? My internet connection has dropped three times today?

I have a Dell computer with Windows Vista and my internet connection is DSL. My modem is by "VisionNet" and it's a wired modem. (NOT wireless). All the cables are securely plugged in to the back of my modem and my computer, my modem is cool and not heated, and I ran a full scan with Avast! Antivirus and it told me I was clean of viruses. So, anyway, early this morning my internet just cut-out for about 20 minutes or so, then, earlier today around 4:30 it went out again for like, maybe 7-10 minutes. Then, again, a few minutes ago, it went out for about 5 minutes. My internet connection only drops maybe once every week and a half. I have online friends who's internet constantly goes in and out. But, since mine DOES NOT usually do this, especially not three times in one day, I'm worried. Whenever I click my internet icon in the bottom-right, then "diagnose and repair", it tells me that it can't communicate with the primary DNS server. Early this morning when it happened, the INTERNET light on my modem was blinking on and off. All the other lights were solid green. I've called my ISP but my connection came back when I was on the phone, twice, and they said they didn't have enough information to determine a cause. Can anyone help me out here? Thanks in advance!